A Complete DISCOURSE OF THE Nature, Use, and Right Managing Of that Wonderful INSTRUMENT, THE BAROSCOPE, OR Quicksilver Weatherglass. In IV. PARTS. By JOHN SMITH, C. M.  
To which is added, The true Equation of Natural Days; drawn up for the use of the Gentry, in order to their more true Adjusting, and Right Managing of Pendulum Clocks and Watches.  
LONDON, Printed for Joseph Watts, at the Angel in S. Paul's Churchyard. MDC LXXX VIII.   

To the READER, Giving an Account of the First Original of this Instrument.  
THIS Nobe Instrument, the Baroscope, which the Reverend and 
Phylos. Trans. N. 9 p. 155.  Learned Dr. John Beale thinks to be the most wonderful that ever was found out in the World, does owe its first Rise and Original to an Experiment first made bp the Famous Torricelius, an Italian Mathematition, who in order to the making of some new Discoveries in the Mysteries of Nature, took a Glass Tube about three Foot long, and having sealed it hermetically at one End, he filled it at the other with pure Quicksilver, and emersing the open End of it into another Vessel of Mercury, he found that the Quicksilver, which before filled the whole Tube, did now in part sink down into the lower Vessel, leaving in the upper part of the Glass-Pipe, a seeming void and empty space. Which rare and unusual appearance did engage him to a more particular notice of it, and induce him to repeat the Experiment: Which being done in Tubes of divers Lengths and different Boars, he still found that the Mercury sunk down in each of them to near the same pitch, as in the first Experiment, which was to about the height of 29 or 30 Inches, except in Tubes that were shorter than this Measure, in which case, they always remained full?  
Which Experiment being soon noised abroad, it put others as well as this first Inventor, upon the enquiry what the true Cause was, that the Mercury should thus fall down to such a pitch and no lower; as likewise whether or no that seemingly void Space in the upper part of the Tube were indeed empty, as it did seem to be, since according to an old Maxim in Philosophy, There could be no Vacuity in Nature.  
But while these Particulars were under Examination, it was observed by some, That the Mercury in the same Tube did not always keep the same station, but was sometimes risen higher than usual, and at other times fallen down more low, which at length was judged to proceed from no other cause but the differing weight of that Air, whose ponderosity was already supposed to be the true cause of the Mercuries being so suspended, as in the first Experiment: And these Changes in the Height of the Mercurial Cylinder, being in process of time observed, for the most part, to happen also in the same Changes of Air, it being always Highest in Fair Seasons, and Lowest in Foul, it was at last proposed as a very fit Instrument to assist us in Presaging the Future State and Change of Weather.  
Note, That the Honourabe Rob. boil, Esq as Dr Beale tells us, in Phil. Trans. N. 9 p. 153, was the first that ever discovered this Useful Instrument to the English Nation, and therefore does deserve from us all due Acknowledgements for those Benefits which we either have or may hereafter receive by it.   
A Complete TREATISE OF THE BAROSCOPE, etc.  

The INTRODUCTION.  
BAroscopes have heretofore been made up after divers manners, but chiefly three ways: As first, That now commonly used, with a Straight Tube, ascribed chiefly to the Noble boil. Secondly, That with a Tube, whose Top inclines, devised by Sir Samuel Morland. Thirdly, The Wheel-Baroscope, invented by the Ingenious Mr. Robert Hook (and described in his Micography) but of these three sorts, the two last are but seldom used, by reason of some Inconveniencies either in the Shape or Charge; Sir Samuel's being such as will not admit of any Regular Figure, and Mr. Hooks being very Dear and Costly. 'Tis true, these do both of them manifest the least Motions of the Mercury more visibly than that with the Sreight Tube, especially the Latter; but this is found to be no great Advantage, in regard that such nice Motions are but seldom followed with any considerable Alteration in the State of Wether: But for those Motions of the Mercury, as are indeed true Tokens of any great Change, to Wet or Dry, these are sufficiently manifest in the Common Baroscope, or that with the Straight Tube; which being also most Regular, of smallest Price, and most easy to be made and managed, shall therefore be the Subject of the following Discourse.   

PART I.  
Of the several Parts of the Common Baroscope, and the most Convenient and Proper Dimensions that are to be observed in making of it up.  
THE Baroscope or Quick-Silver-Weather-Glass is composed and made up of the following Parts.  
First, The Frame, or main Body of the Instrument. Secondly, A Glass-Tube or Pipe. Thirdly, A Cistern or Receiver. Fourthly, Two Register-Plates, with a Sliding-Index. And Lastly, A sufficient Quantity of Quicksilver to fill the Tube, and adjust the Receiver. Of these I shall treat in their order.  

Of the Frame.  
The Frame is Wood, of which any sort may serve, but for Ornament sake, the Choicest are generally made use of, such as Ebony, Walnut or Olive-Wood. The Shape and Figure is various, according to the different Fancies of them that do either make or use them; but for the Size, that must always be such, as that the Length may admit a Glass-Tube, of at least three Foot Long, and its Breadth sufficient to affix thereon the Register-Plates.  
In the middle of this Frame must be cut out a halfround Grove or Channel, throughout the whole Length of it, and of such a Depth, as may be sufficient to secure the Tube, when set in it, from being Broken by any outward Accident; for which Reason this Grove ought always to be made so Deep, that the Tube may rather stand within, than any ways without the Wood of the Frame, for by this means it will be the better Secured from Danger. But some, to secure the Tube more effectually, add a Cover of Wood, which does effectually prevent those Dangers, to which the Glass is at any time subject.  
Near the Bottom of the Frame is to be affixed the Cistern-Box and Cover of such a Size and Bigness as may admit a Glass Cistern of three Inches Diameter, or three and a half, and one Inch in Height, at least.  
Lastly, Upon the upper part of the Frame are to be affixed the two Register-Plates. in doing of which you must observe a due Distance between the Lowest Division on the Registers, and the Bottom of the Cistern-Box; and for this the Rule is, To set the one just 28 Inches and a half distant from the other. I know some are not willing to admit above 28, and a quarter Distance, because less Mercury will then serve to adjust the Receiver; but this is too little in Reason. We think it most requisite to allow a quarter of an Inch more, (some allow more than this) that so there may be a greater Depth of Quicksilver to immerge the open-End of the Tube in, for the Deeper this open-End of the Glass is buried in the Quicksilver, the better will the Air be prevented from any ways getting into the Boar of the Tube, which in the long standing of some Tubes, whose open-End has been but slightly buried in the Mercury, it has been found to do; for Air is so subtle a Matter, that it will be apt to insinuate itself by degrees into the Tube, in spite of all the Quicksilver within it, unless there be a sufficient quantity without, to prevent its depressing itself so low as to get in at the open-End: And tho' our Senses perhaps cannot perceive this, yet the Glass itself will discover it at last, who in a few Years standing will not move so Regular as at first, by reason of the expanding and contracting of that Air that has thus insensibly got up into the Head of the Tube.   

Of the Tube.  
The Tube is a strong, evenblown Pipe of clear Glass three Foot Long at the least, and the Boar of it not less than two Tenths of an Inch, nor greater than one quarter of an Inch at most in Diameter; let it be sealed very close at one End with the Hermetick Seal, and filled very true and even at the other, yet somewhat asloop.  
Note, That if these Circumstances are not all observed in it, the following Inconveniencies will certainly ensue.  
For First, The Boar must not exceed one quarter of an Inch; for if the Boar be larger, it will cause the Mercury in the Cistern to Rise and Fall too much, when that in the Tube does on the contrary either Fall or Rise (except due proportion be observed). And on the other hand, if the Boar be smaller than two tenths, it will then be extreme difficult to fill it with Mercury, and more difficult to purge it from Air when you have done so.  
Secondly, Unless the Tube be well sealed at the closed End, the Mercury will never be kept suspended, but will still sink down into the Receiver, when ever you attempt to set it up for use. Now to know whether the sealed End be perfectly closed up or no, do thus immerge or dip the sealed End into a Vessel of Water, and then blow strongly at the other End with your Mouth, and if the Tube be not perfectly closed, you will perceive Bubbles to rise up through the Water, which Bubbles will not be found if the Sealing be firm.  
Thirdly, Unless the open-End of the Tube be filled or ground very smooth and even, after it is first fitted to a Length, it will be very difficult, when filled with Quicksilver, to put it up in the Frame it is to stand in, without the loss of much Mercury; for if the Tube have the least Gap or unevenness left in it, it will be very difficult to stop it so close with your Finger as to keep the Mercury from spurting out thence, when the Tube is filled, in order to be set up, for Quicksilver of itself is of so strangely a fluid Nature, that when it is so pressed with its own weight, as it is in a full Tube, when thus elevated upright, in order to be set up, that it will make its way out at the least Cranney imaginable.  
But though it requires to be thus even and smooth at the End of the Tube; yet this aught to be so ordered, that the End be not perfect flat, but rather somewhat asloop, that so when the Tube is set up in the Cistern, some part of its Edge may rise from the Bottom, and not touch it on every part; by which means the Mercury will have the freer liberty to pass in and out, as it does rise and fall in the Tube above.  
Note, That in cutting a Glass-Tube to a due length, there is way to do it with ease and speed, and that is by first filing a small Notch in one Side of the Tube, and then laying that Notch over the Edge of some Iron, you may by striking a little distance from it with a Hammer, on that part to be broke off, break it with ease: Which method is so certain and safe, that by it I have broken a piece of a Tube, a foot long into twenty several bits, without ever missing. This being a Novelty of some use, I thought fit to insert it here.   

Of the Cistern or Receiver.  
The Cistern is a round Vessel of Glass, about three quarters of an Inch deep, and three Inches in Diameter, at least, or three and a half, and perfect flat at bottom. I know there be few yet made of this Size, neither will people be willingly persuaded to admit of this bigniss, in regard that it requires much Quicksilver to fill it its due depth, which being a dear Commodity, Men strive rather to make as little serve as they can, that so the whole instrument may be afforded more cheap by them that sell them.  
But for such persons as mind more the perfection of a Weatherglass than the cheapness of its price; such must be no-wise unwilling to admit of this magnitude, for in this one particular does the whole Perfection of the Baroscope consist, for the larger the Cistern is in Diameter, the more certainly may the Glass be adjusted at all times, and the more exactly will the Mercury in the Tube both rise and fall. And this I shall endeavour to make plain in both particulars.  
First, As to the true adjusting of a Glass at all times, the more certainly, by means of a large Cistern: This I think will appear, by showing the uncertainty of performing this, when the Cistern is small; For, suppose a Glass be set up, and adjusted, at a time when the Air is of so small a weight, as to suspend the Mercury no higher than 28 Inches and ●; and this same Glass were after a while to be taken down, and new set up, at such a time when the Air is so weighty, as to buoy up the Mercury to 30 Inches and ● high; I say then, That this Glass, which before was well adjusted, when the Air was light, shall now, when the Air is thus much heavier, require so much more Quicksilver to adjust it, as is contained in the Tube between the Figures of 28 ½, and 30 ½, for so much Quicksilver being now forced up into the Tube, will sink that in a small Cistern very considerably, and so much as is forced out of the Cistern into the Tube, so much must be added to the Cistern to make that Mercury in it rise up, till it be within its due Distance of the Register-Plate. Now it follows, That if more Quicksilver be required to adjust a Weatherglass, when the Air is heavy, than there does when the Air is light; then those Glasses that have such small Cisterns, cannot be good, for in a good Glass the same Quantity of Mercury that does once adjust it, will be sufficient to adjust it at any time after; for in a good Baroscope, let the Air be Heavy or Light, or the Quicksilver High or Low in the Tube, yet the surface of the Mercury in the Cistern ought always to keep the same Distance in Inches from any of the Figures on the Register-Plate, as the number of the same Figure does import: Now if it must neither Rise nor Fall in the Cistern, it follows, That the Cistern must be always so proportional to the Tube, that the greatest Rising and Falling above, must not any whit alter the surface of that below.  
Secondly, As to the more exact Rising and Falling in those Tubes that stand in large Cisterns, beyond those that stand in small ones. I think this may be plainly understood, by considering the Motion of a Glass, at such a time as the Air by becoming more Light, would naturally suffer the Mercury to sink down in the Tube one whole Inch: In which case, suppose for demonstration sake, That a Cistern were so very small, that this Inch Falling of the Mercury in the Tube, shall raise that of the Cistern, in which it stands, one complete quarter of an Inch: From hence it will then follow, That when the Quicksilver should naturally have fallen in the Tube one whole Inch, yet it will not now appear to fall above three quarters, for falling about three quarters above, and rising almost a quarter below, will make the two Mercurial Surfaces to be now but 29 Inches asunder, that before, we supposed, were ●0, which is equivalent to the falling of an Inch of Mercury: Whereas, had the Cistern been so large as not to admit of any rising, there then, the same Mercury in the Tube, must have fallen one whole Inch, before the two Mercurial Surfaces had been altered an Inch in Distance; from all which, it is plain, That one Glass with a small Cistern, may fall but three quarters of an Inch, when another with a very large one, shall at the very same time fall a whole Inch.  
And this shall suffice to demonstrate the Necessity of the one, and the Uncertainty of the other, and what Care there ought to be taken, so to Proportion the Tube and Cistern, That the greatest Rising and Falling Above, shall not at all alter the Mercurial Surface Within the Receiver.  
Now that you may Fit any Tube with a Cistern in this Proportion, do thus: First, Make a Cistern-Gage, which need only be a Pin driven into the straight Edge of a Ruler, and then cut off to such a Length, That when the Ruler is laid cross the Brims of the Cistern, the Pin may reach down to near the Middle of its Depth, or so Deep as you Judge the Mercury will Rise when the Instrument is jousted.  
Then take the Tube you intent for use (being first fitted to the Frame), and empty, as it is, put it up in its place, and with a Pencil make two Marks upon it, just against the Divisions of 28 and 31, on the Register; then take the Tube from the Frame, and fill it with Quicksilver quite full, stop it in with your Finger, and turn the Sealed End upright, and when you have so done, let down the Open End, so stopped, into a deep Earthen-Vessel, and there let the Mercury run out gently till it be sunk down in the Tube so low as the first or uppermost Mark made thereon; at which making a stop, let the Mercury included between these two aforesaid Marks, run out into another Vessel by itself, and when it is sunk down to the lowest Mark, make a stop, and let the rest run out into the first Vessel, or dispose of it otherwise, at your own pleasure.  
Then let that Mercury contained between the two Marks aforesaid, which was let run out into a Vessel by itself, be weighed exactly, and let the Weight be set down in a Paper, that so you may know again how to make out the same quantity on occasion, or you may let that Matter remain in the opposite Scale, that did there counterpoise the Mercury so weighed.  
When you have so done, then lay the Gage across the Brims of the Cistern, and fill the Cistern with Mercury, till it just touch the Top of the Pin, then put in also, all that Mercury, which before was weighed; or so much as equivalent to the Weight of it set down or reserved in the Scale; and if it does not raise the Surface of that Mercury, already in the Cistern, any whit sensibly, then is the Receiver fit for a Tube of that bigness, in regard that its Size is so large, that the whole Quantity of Mercury contained in the Tube, between these two Marks, answerable to its highest Rising and lowest Falling, (when put in) does not any whit sensibly mount the Surface of that Mercury within itself.  
But if you find, by the help of the aforesaid Gage, that this quantity of Quicksilver, when put in, does raise the Cistern Mercury any whit sensibly above the Point of the Gage-Pin; then according to the former Method, try a Cistern somewhat bigger, and if that does not fit, then try a third, and continue so to do, till you have found out one whose Size is sufficient for the Tube you intent to use.  
He that observes this Rule, may exactly fit a Cistern to any Tube, and make it Proportional, let it be of what Size or Shape soever it will.  
Note, That according to the foregoing Rule, a Tube whose Boar is two Tenths of an Inch in Diameter, will require a Cistern of three Inches over: And a Tube of a quarter of an Inch Boar will require one of three Inches and a half at least. By which you may perceive how defective most Baroscopes, already made, are in this point of Perfection.   

Of the Register-Plates and Sliding-Index.  
The Registers (so called, because they continually give an account of the Height of the Mercurial Cylinder) are two Brass Plates of about six Inches in Length, and an Inch and a quarter broad; in the middle part of which, on that Edge that respects the Tube, are graduated, the divisions of three whole Inches, with Decimal subdivisions, and at the four Master-Lines, that bound these three Inch-Spaces are engraven on one of the Plates the Figures following, viz. at the Lowermost 28, at the next 29, at the next to that 30, and at the Uppermost of all 31. Now the use of these Divisions thus numbered, Is to show how high, or how many Inches the Quicksilver in the Tube does at any time surmount that Mercury in the Cistern below it.  
In this first Register-Plate is also cut out a Grove or Slit, somewhat longer than the Graduations, in which is to be fitted a Sliding-Index, whose use is to denote, both whether and how much, the Mercury does either Rise or Fall within the Tube.  
The other Register-Plate is also divided and subdivided, as the former; yet not numbered with Figures as the other is, but instead thereof, there is engraven thereon certain Words, which are designed to denote those usual States of Wether, which Learned and Ingenious Men have observed for the most part, to follow when the Mercurial-Cylinder is in Height equal to any of those particular Divisions to which those Words are are affixed; the Words themselves being these that follow, viz. at 28 is engraven Great Storms, at 28 ½, Much Rain, at 29, Rain, at 29 ½, Uncertain or Changeable, at 30, Fair, at 30 ½, Settled Fair, at 31, Very dry.   

Of the Mercury, with which the Glass is filled.  
As for the quantity of Quicksilver, which each Baroscope does require, no certain account can be given of that, in regard of the various Sizes of both Tubes and Cisterns; in which respect several Quantities may be required. A good Baroscope may take up from two Pound, to three or four Pound of Quicksilver, according as it is in Size and Goodness.  
But note, That in this Particular, the greatest Point is carefully to choose good Quicksilver; for Quicksilver is sometimes adulterated with Tinn, which makes it naught for this Purpose; it being thereby made less fluid, and so, not apt to rise and fall with that freedom that is required in a good Baroscope.  
Now if you suspect a Cheat, which you may partly guests at sometimes by the Eye; for adulterated Quicksilver looks somewhat dull, and is commonly crusted over with a kind of Creaminess: whereas good Mercury looks bright and shining. But there be some so cunning at this knack, as to adulterate it so completely, that the naked Eye shall not perceive it.  
Now to discover this cunning cheat, do thus; Take about the quantity of a Pease (or more) and put it into a clean Silver Spoon (an old Silver Groat, made hollow, may do as well) and set it over some hot burning Coals, in a Chafingdish, and in a little time the heat of the Fire will make all the Mercury fly away in Smoke; and, if when it is all flown away, it leave only a White or Yellowish Spot behind it, it is then fine and good, but if there be left behind any kind of Substance that will not evaporate, or if it slain the Silver of a Black or Dark colour it is then bad and adulterated Quicksilver, and no ways fit to fill a good Baroscope.  
And thus much I think sufficient to be noted concerning all the chief parts of the Baroscope, or Quicksilver Weatherglass: As for the true Shape and Figure of each particular, as likewise the way of its being made and fitted by manual Operation, these I have thought fit to omit, as not thinking it worth the while to describe that, which every one that makes of them does already understand, and which if it should be attempted, is not well to be done by Words: All that has been done hitherto, having been done only in order to make Men more rightly to understand the true Nature and Perfection of the whole Instrument.    

PART II.  
Directions for the right Ordering of a Baroscope, in the setting of it up.  
AS for the Packing up a Baroscope, in order to its being conveyed to some far Distant Place; this I shall omit, as leaving that to the Workman's care: Who, when he has fixed the Instrument in all its Parts, and filled it with a sufficient quantity of Quicksilver, will also take care to pack it up well. All therefore that I shall now discourse of, is, What is to be done in the Workman's absence, or at a Mans own place of Residence, at a distance from him.  
Having therefore first unpacked every part of it with care and caution, provide to the setting of it up in the manner following. First, set up the Frame, and let that be fixed fast to the place, where 'tis designed to stand for use; which is done by first driving therein some Nail or Tenter-hook, upon which the upper part of the Frame may be suspended, driving in another Nail just under the Bottom, to which fasten some String or Ribbon, which being tied about the Bottom-Button of the Frame will keep all firm and steady.  

How to Cleanse and Prepare the Tube.  
When the Frame is thus fixed fast in its place, then take the Glass-Tube, and make the inside of the Bore both dry and clean, for otherwise the Mercury will never play free therein, but hang to the Sides of the Glass; neither can it be perfectly cleansed from Air or Blebs of Wind.  
To do this, Take some Brass or Iron Wyer, about the Size of a great Pin, and about a handful longer than the Boar of the Tube, and having made it straight, file one End of it taper and sharp at the Point, and afterwards turn it in the fashion of a Worm, like those with which Men draw out the Charge of a Birding-Piece: This Worm, so form, screw into some clean dry Rag, and putting of it into the Boar of the Tube, draw it therein backward and forward from one End to the other, till you judge the Glass to be thereby made perfectly dry and clean; then clear the Tube well on the outside with a dry Cloth, and set it by.  
Note, That if there be the least Dust or Lint left behind in the Tube, there will a small Bubble or Bleb of Air gather about that part, in spite of all the care and pains that shall be used to get it out thence; and no Art can perfectly cleanse it from Air, till whatever offends be gotten forth from thence.  
Note also, That for such as are not capable of making a Worm, these may only turn back the End of a Wire (first heating of it red hot in the Fire to neale and soften it) and make therewith an Eye, like that of a Needle, through which a Rag may be drawn, and this will serve almost as well as the former.   

How to order the Quicksilver.  
If, by the foregoing Method of examining the Quicksilver by Fire in a Silver Spoon, you find it pure; than you have no more to do, But to take care to cleanse it from Dust and other Filth, which is done by straining of it through some new piece of Fine Linen Cloth, or (which is much better) through a piece of Fine Shammey, or thin Wash-Leather Sheepskin; for this will suffer no Filth to pass through it, neither will it foul the Quicksilver with its own Substance, as Linen is apt to do by its own Lintiness.  
Note, That in straining the Mercury, you ought not to use about that Work any Vessels of Tinn or Pewter, for the Mettle of such Vessels will dissolve in the Quicksilver and spoil its Goodness; use therefore in this Work only Earthen or Wooden Vessels.   

How to fill the Tube, and Cleanse it from Air.  
When the Mercury is well cleansed, then take the Tube, and resting the close end of it in some large Woodden or Earthen Bowl or Platter, to save the Quicksilver, should any spill in the filling; then grasp the open End of it in your Hand, between the root of the Thumb and Forefinger, somewhat low, that the hollowness between these two may serve instead of a Tunnel, then fill the Tube with Quicksilver (by either pourring of it out of a Glass-Viol, or else by taking of it out of some other Vessel with the clean Bole-end of a Tobaco-Pipe) till it want somewhat less than a quarter of an Inch of the Top.  
Then to cleanse it from Wind, Stop the End with your Finger, and by raising up the sealed End of the Tube to somewhat above a level, let the Air included in that space you left empty, rise up very gently towards the Sealed End, and this will take into itself all those Blebs of Wind that lie in its way, and when this Bubble of Air is risen up quite to the End, let it return again, by depressing a little that End, which before was upmost, and turning the Tube a little at the same time, that the Bubble may take in its way other Blebs of Wind that yet may remain, and thus turning the Tube each time, you let the Bubble pass and repass from one End to the other; it will in a little time cleanse the Cylinder of Mercury from all those Blebs of Wind that appear either by the Sides of the Glass, or are included in the body of it.  
Note, That if, when you first fill the Tube, you leave the space of half an Inch or an Inch unfilled, as some do, you will find it then ten times more hard to cleanse from Air than if you leave it empty not above one quarter of an Inch space.  
Now, when the Tube is so perfectly well cleansed from Wind, that the whole Cylinder of Mercury does appear in the Glass, as if it were a perfect smooth and well polished Body, and free from the least Speck of Air, then turn up the open End of the Tube, and fill it quite full with Quicksilver.  
But, if, as I noted before, the least Lint, Hair or Dust imaginable, stick to any part of the inside of the Tube, the best art of Man cannot then free it from all Air, but some will gather still about this Matter that offends, in spite of all our Endeavours: In which case, there is no Remedy but to cleanse the Tube anew, according to the Method before laid down.   

How to put up the Tube in its Place, within the Cistern or Receiver.  
Wipe the Cistern very clean, and set it within the Cistern-Box; then put therein as much Mercury as may fill it half full, if it be a Glass that has not yet been adjusted; but if it has been adjusted already, and fitted with its due quantity of Mercury, then put into the Cistern only that part of the Mercury that remains when the Tube is filled quite full.  
Then take the Tube, and stopping the End of it with your longest Finger, invert it so, that you may immerge the open End of it with ease into the Cistern-Mercury, in such a manner, that both Glass and Finger may touch the Cistern-bottom, keeping the Tube, in the mean time, as upright as you can; then of a sudden, take away that Finger that stops it, and as suddenly, at the very same instant of time, raise the Tube just upright; which if you perform nimbly and well, will be done before the Mercury in the Tube is all sunk down to its lowest Station. Now, if the Tube be raised almost upright, before the Quicksilver be all descended, you will not fail to set it up completely at the first trial.  
But in case any Air should be admitted into the Tube in your first attempt, then must it be taken down and repurged a second time, and again set up, according to the Method before described, and if you should fail this second time, then try a third for Experience will at length make you perfect, and enable you to do this Work complete and well: But note, That a Tube is never well put up, till, according to the former Method, you have put it up so, as that no Wind has found admittance while the Work is doing.  
When the Tube is well put up and fixed in the Frame, observe, Whether the Cistern stand well, for some Cisterns are so placed in the Box, That the Tube, by Reason of its nearness to the Rim, cannot be well surrounded by the Cistern-Mercury, by which means, the Air does many times find admittance that way, and so gets up into the Head of the Tube, and spoils the Glasses true Motion.   

How to Adjust the Instrument when the Tube is set up.  
If the Glass be already adjusted to your hand, by the Workman that makes it (as I hinted before) than you have no more to do but to put all that quantity of Quicksilver into the Cistern that does remain after the Tube is filled quite full; and then when the Tube is well put in it, your Baroscope will be well adjusted, without more ado; for when a Weatherglass is once fitted with a due quantity of Quicksilver, that same weight or quantity will be always sufficient.  
But, in case the Glass be not already adjusted, or that, by mischance, some part of the Quicksilver is spilt and lost, you must then proceed to adjust it according to Art, which is thus done: Take a straight Rod or Rule of Wood, of the just length of ●8 Inches, this we call the Justening Gage; put one End of this Gage into the Cistern, and raise up the other End, so that it may just touch the lowest Division on the Register, or that against which the Figure 28 stands, and keeping it there fixed in that Posture, with your Finger, observe whether the Quicksilver, in the Cistern does just touch the lower End of the Gage, which if it does, then is it already adjusted well; but if it wants somewhat of touching the End of the Gage, then put in more Mercury, till it does so, but on the contrary, if there be already too much Mercury in the Cistern, which is known by its rising above the End of the Gage or Ruler, then with the clean Head of a Tobaco-Pipe take out so much of it as may sink it down just so low as the End of the Gage; and then is the Glass completely and well adjusted.  
Now the adjusting of a Baroscope, being nothing else but to raise the Quicksilver to such a pitch in the Cistern, that its Surface may be distant from any Figure on the Register the same number of Inches as the said Figure specifies; it follows, that any other Figure will serve to adjust it by, as well as that of 28; for a Rule or Gage of 31 Inches long will do as well, if applied to the Figure 31, as one of 28 applied to the Division of the Figure 28; for when the Glass is well adjusted, the the Quicksilver in the Cistern is 31 Inches distant from the Figure 31, as well as 'tis 28 Inches distant from the Figure 28, and the same is to be supposed of 29 or 30.  
Note here, That when a Glass-Tube is well fitted with a Cistern and well adjusted, then if you measure at any time the height of the Mercurial Cylinder, you will find it always the same, as the Division on the Register notes, against which the Quicksilver stands: But if the Cistern be too small in Proportion then the Cistern-Mercury will either rise above or fall below the Pitch to which 'twas at first adjusted: In which Cases, the aforesaid Measures will not be found to agree. And this may serve as another Instance of the Necessity of a Large Cistern.   

How to Remove a Baroscope to some small Distance.  
If it be only to remove it out of one Room into another, you need do no more, but loosen the Frame and take it from the Place to which it is fastened, and so, upright in the same Posture it stood in, let it be carefully conveyed from one Place to another by Hand; but let this be done gently, and with a careful step, lest the QuickSilver surge over the Sides of the Cistern.  
But if there be occasion to remove a Baroscope, by Hand, some larger Distance, as a Mile or two, then do thus: Loosen the Tube from the Frame, and gently incline the Head of it, till the Mercury fill the empty space in the Head of the Tube, then nimbly slide your Finger under the open End that stands in the Cistern, stop it up close, and lift the whole Tube full of Mercury, as it is, quite out of the Cistern; then turn the open End up, and take out from thence, as much Mercury, as will somewhat more than admit of a Cork, stop it up close, then empty what Mercy remains in the Cistern, into a clean Viol-Glass; set the Cistern again in its Box, put therein 6 or 8 doubles of a Linen Cloth, and upon that replace the full Tube, with the sealed End downwards and the Cork up, and having fastened it in the Frame, take Frame and all down together, and convey it, by Hand, to the Place intended; which you may do with great Ease and Safety, for the Tube, though full of Mercury, will not be at all in Danger of Breaking, so long as 'tis kept firm in the Frame. When you are come to the Place, where 'tis to stand, set up the Frame in its Place; then take out the Tube, unstop it, and fill it quite full; then put all the rest of the remaining Mercury into the Cistern, set up the Tube in it, according to the Method before described, fasten it well, and the Work is done.  
Note, That by this way you will save all the Trouble of cleaning the Tube, straining the Quicksilver, and purging the Mercury from Air, when the Tube is filled.   

How to Rectify a Baroscope, that by long standing, does begin to move disorderly.  
It being so difficult for some persons to purge some Baroscopes from all kind of Air at first, it follows, That the Air, which has gotten into the Tube at its first filling, and is not perfectly purged out from thence, will in time get free, and ascend into the Head of the Tube, which, if it amount too much, 'twill in time greatly alter the Glasses Motion, by expanding and contracting itself with Heat and Cold; to remedy which, Baroscopes ought to be sometimes rectified, and when this is needful, may be known thus: Incline the Head of the Tube from the Frame, without lifting of it from the Cistern, till the Head be filled by the Mercury's rising up in it, and if you perceive a Bubble of Wind at the top, 'tis a sign that it wants Rectifying.  
Now to do this, You need only incline the Head of the Tube till it be full of Mercy, and then sliding your Finger nimbly under the End of it in the Cistern, and stopping it close, take the Tube out, full of Mercury, and so expel the Bubble, by depressing the sealed End so low, that the Bubble may mount up, and pass out at the open End.  
But sometimes, if the Bubble be too small, 'twill then remain immovable, and will not be made to mount up, unless some more Air be let into it, which you may do by emptying out a little Mercury at the open End, and so stopping it with your Finger, let that Air run up and join with the Bubble, and then they will with ease return both together in one Body; which being done, fill the Tube and set it up again in its Place, and the Work is done.  
Note, That a Baroscope thus rectified once or twice in a Year or two Years time after 'tis first set up, will scarce need it any more afterwards, let it be kept at work as long as it will.    

PART III.  
Of the Uses of the Baroscope, or those Rules, by which we are to Judge what will be the Future State and Change of Weather.  
THe chief Uses of the Baroscope are, To foreshow Fair and Foul Wether, (but not Heat and Cold, these indeed are sometimes judged of by consequence, but are not at any time precisely indicated by the Glasses Motion:) Now that you may be the better directed, which of these to expect, You must still note, the Rising and falling of the Quicksilver; for the Rising of the Mercury, let it be in what part of the Glass soever, does betoken Fair Wether; and, on the contrary, its Falling down from any Place, where it last stood, does foreshow a Foul and Wet Season.  
In order, therefore, to discern all these various Changes of the Mercury in the Glass or Tube, you must still follow the Motion of it with the Sliding-Index, and set that continually, from time to time, just equal to the Top of the intubed Mercury, by which Means, you will have always an exact Account how the Mercury does at any time alter; that is, Whether it does either Rise or Fall, or how much the quantity of that Rising and Falling is, for the Index still keeping the Place, where you set it last, 'twill still show you which way, or how much the Mercury is removed from its last Height or Station.  
Note, That as to the Figures on the Register-Plates, their Use is only to denote, How High the Mercury, in the Tube, stands above the Surface of that in the Cistern, and as for the Words thereon engraven, they are not to be noted, except the Mercury be moving from Changeable either Upwards or Downwards; but when it does so, than the Words are of Use; for if the Mercury be moving Upwards from Changeable, than the Wether will, for the most, be such as the Words in that part of the Register do denote. On the contrary, if the Mercury be already below Changeable, and does yet fall down Lower, than the Words in that lower part of the Register may be noted; for the Wether, will then, for the most part, fall out accordingly.  
But, if the Quicksilver does Rise when 'tis below Changeable, or fall when 'tis already above it, than the Words signify nothing; for if Rising in any Part signifies Fair, and Falling in any Part does denote Foul, than it follows, That if it fall from Settled Fair to Fair, it may Rain, notwithstanding the Quicksilver does still stand as High as Fair. The like may be said if it Rise from Much Rain to Rain, for its Rising does now betoken Fair Wether, notwithstanding 'tis still as Low as the Word Rain: Therefore take this for a general Rule, That if the Mercury move Upwards from Changeable, or fall from Changeable Downwards, then will the Wether be for most part such as these Upper or Lower Words denote; but if it either Rise towards Changeable, or fall downwards to it, when 'tis already above it, than the Words signify nothing.  
If Wet and Rainy Wether come presently upon the Mercury's Falling, and the Mercury does at that time fall but a little, then expect not much Wet at that time, for the Rain will not then hold long; but if the Quicksilver falls very much in a little Time, or if, when it begins to Sink, it continues to do so for two or three Days together, before the Rains come, then expect very much Wet Wether after it.  
On the contrary, If Fair Wether follows immediately upon the Mercury's Rising, and the Rising be also but a little, then expect not much Fair Wether at that time; but if it Rise very much on a sudden, or if, when it begins to Rise, it continues to do so for two or three Days together, before the Fair Wether comes, and the Rains cease, then expect much Fair weather to follow.  
But in all these foregoing Cases, the Wind must well be noted; for unless you do that you may chance to be sometimes mistaken, for tho' the former Rules are for the most true and certain, and seldom fail, yet sometimes they are found so to do; but when this does chance to happen, you will, for the most part, find it to do so at such times as the Wind sits in such Quarters, whose temper is contrary in Nature to that Wether which the Glass Predicts; for if the Mercury fall, which is a Sign of Wet, yet if the Wind, at the same time does blow from the East or North, it may still continue Fair, because, these for the most part are dry Winds, and seldom produce Rain, for Reasons hereafter given. On the contrary, If the Quicksilver Rise, which signifies Fair, yet, if the Wind be then Southerly, it may chance to Rain for all that; for Southerly Windle are generally Moist, and many times bring in gross Vapours that are too heavy to be suspended by any weight of Air which they produce.  
But if the Mercury fall, and the Wind be Southerly, 'tis a thousand to one but Rain will follow; and on the contrary, If the Quicksilver Rise, and the Wind be Easterly, 'tis a thousand to one, but 'twill then be Fair Wether, for in these Circumstances, I have seldom or never, found the Glass to fail; let the Wind, therefore, be noted well in every Case.  
If the Mercury be very high in the Summertime, when 'tis Hot Wether, and does of a sudden fall down a considerable Distance, then expect Thunder, or great and sudden Storms of Rain.  
If the Mercury does Rise very High in the Wintertime, viz. to Settled Fair, or above it, and the Wind be then North or East, it does certainly presage much Frost, and you may be certain also, That the same will continue as long as the Mercury does stand thus High. But when the Mercury in a settled Frost Sinks down suddenly five or six Divisions on the Register, 'tis then a certain sign of a Thaw, and the Frost will then undoubtedly break up and be gone.  
If, in a Frost, when the Mercury has stood High for a time, it does of a sudden Rise yet Higher, than you may certainly expect Snow to follow.  
If the Mercury Rise and fall but a little at a time, and it be very Unsettled its Motion, it than argues the Wether to be Unconstant, and that it will not continue long in one state.  
Note, That falling, when the Mercury is already below Changeable, is a sign of more Wet than falling above it: And on the contrary, Rising, when the Mercury is already above Changeable, denotes more certain dryness, than Rising does below it.  
As for Heat and Cold, that is gathered from this Glass by consequence only, for when the Mercury stands High in Summer, as it denotes Fair Wether or very Dry, so by consequence it argues a Hot Season. So likewise, if it be extreme High in the Winter, it argues much Cold, for height of Mercury, being at this Season a certain sign of Frost, it follows, that Gold will undoubtedly attend it.  
Note, That the Mercury is ever lowest of all in extreme High and Strong Winds, but yet it does no way predict Winds before hand, as it does most other Wether; for the Mercury falls thus extreme low, at the very time only of the Winds blowing. Note also, That as soon as these High Winds cease to blow, the Mercury then generally Rises very fast of a sudden; but such Risings that do immediately follow Storms of Wind are no certain Signs of Fair Wether, unless it Rise to above Changeable.  
Note also, That the greatest Heights of Mercury are always found upon Easterly or North Easterly Winds, and and that the Fairest Wether and Driest Times are, in this English Nation, always accompanied with these kinds of Winds.   

PART IU.  
Giving an account of the most probable Reasons of the various Motions of the Baroscope, and the Effects that follow.  

The Reason in general of the Mercury's Suspension in the Tube.  
'TIs now generally agreed upon, on all sides, That 'tis the pressure of the Air upon the Stagnant Mercury in the Cistern, that does buoy up that in the Tube to its usual Hight; but in regard 'tis hard for some to conceive the manner how, I shall endeavour to demonstrate it in a Cyphon-Tube, which, for this purpose is much more proper than a Common Weatherglass.  
Take then a Glass Tube or Pipe, about four Foot long, seal it close at one End, and let about a Foot of the other End be turned up, Cyphonlike, in the Form of a Fish-Hook, fill the longest Leg of this Tube with Quicksilver, cleanse it from Air, and then raise it upright, upon which you shall find that the Quicksilver will Sink down in the Longest Leg, and Rise up in the Short One, till the two Mercurial Surfaces are equally Distant with that of the Tube and Cistern in the Common Weatherglass.  
Now, the Reason of its being thus Suspended in the Longest Leg does proceed from the Airs pressure at the open End of the Short One; for suppose that a Cylinder or Pillar of Air, of the bigness of the Boar of the Tube, be extended so high, as to equal in weight a Cylinder of Quicksilver 29 or 30 Inches High, which some conclude to be near 45 Miles; it follows, That the Weight of this Pillar of Air pressing in at the open End of the Tube, will balance or suspend that Mercury that is just equal to itself in Weight; and from hence it is, that as the Air does grow either Heavier or Lighter, so will the Quicksilver in the Tube either Rise or Fall.  
Now what is here said of the Cyphon-Tube, is to be understood of the Common-Weather-Glass, for in that 'tis the Pressure of the Air on the Cistern-Mercury that buoys up the Quicksilver in the Glass-Tube, and according as this Weight of Air does alter, so does the Mercurial Cylinder either Rise or Fall, and from the Weight of Airs being the Cause of its Motion, it comes to pass, That Baroscopes which stand in High Mountains never Rise so High as those that stand in lower places, which may, in great part, suffice to prove, That 'tis the Airs pressure that does suspend the Quicksilver, as aforesaid.  
This last particular has been experimented by divers Persons, but especially by Dr. Power, who in the Year 1653, found that at 
Powers Exp. Philos. p. 104.  the bottom of Hallifax-Hill, the Mercury stood at the pitch of 29 Inches high, but conveying it from thence to the top of the Hill, he found it to fall there near half an Inch lower. The like is reported of Dr. Pascal, who upon the top of a Mountain 500 Perches high, near Claramont, found the Mercury to stand three Inches lower than it did at the bottom. The like Remark has been made by Esq boil, who tells 
Phil. Trans. N. 11. p. 182.  us, That a Baroscope standing at Stanton St. john's, 4 Miles distant from Oxford, on a Hill, was usually between 2 and 3 eights of an Inch higher than one at Oxford that stood lower near the Thame-side. All which can proceed from no other Reason, but from the various gravity of the superincumbent Air; for a longer, and so consequently a more weighty Column of Air does press upon the Cistern-Mercury in Low Countries than upon the tops of Mountains, which being nearer the top of the Atmosphaere, has by consequence a less weight of Air to press upon it: So that from hence we may conclude, That as it is the weight of Air that suspends the Mercury in general, so it is the differing degrees of that weight that makes it to Rise and Fall.   

By what means the Air becomes sometimes Thinner and Lighter, and sometimes more Thick and Heavy, than usual, in this Nation.  
These, according to Mr. Ed. Halley's Opinion, in the Philos. Trans. of May 25. 86. are supposed to arise from those contrary Winds that usually blow in these Northern Climates; for if two contrary Winds blow towards the place where the Baroscope stands, the Air is thereby increased both in Height and Weight, and so made capable of supporting a greater Pillar of Mercury; as for instance, if it blow a Westerly Wind in Ireland, and an Easterly Wind in some parts of Holland and Flanders, this would crowd in abundance of Air over England, and heap it up to a great Height, and by consequence, make the Weatherglass Rise much; on the contrary, two Winds blowing from the place where the Weatherglass stands, will exhaust the Air, and by consequence make it more Thin and Light there, and that will cause the Quicksilver to sink; as for Example, if it should blow a Northern Wind in France, and a Southern Wind in Scotland, or one of these in Holland and the other in Ireland, it must needs then follow, that the Air impendent over England would thereby be much lessened and exhausted, and by consequence made more light, and less able to suspend so great a quantity of Mercury.   

Why Wet and Rainy Wether follows the sinking of the Mercury.  
The reason of this is, because the Air being then grown Thinner and Lighter, is unable to support those Vapours that before were suspended in it, for they being now too Heavy to float in a Lighter Medium, do descend, and in their fall incorporate themselves with others, till at last, by many additions they become Drops of Rain.   

Why Fair Wether follows the Mercury's Rising.  
The Reason is, Because the Air being now grown more Heavy and Thick, does with ease support those Particles of Water which before it could not suspend, for all Bodies swim in that Medium that is specifically heavier than themselves; as is made out in that Common Experiment of making our English Metheglin, where the Rule is, To make it so strong of the Honey, that it may make an Egg swim, and rise up above the Liquor the breadth of a Groat. Now this Egg in plain Water sinks to the bottom, but when the Water is made so strong of the Honey, that the quantity of an Egg of it shall be heavier than the Egg itself, than the Egg swims. Even so here, when the Air is so crowded together by contrary Winds blowing towards the same place, as to be made more heavy than before, than those particles of Water, which before fell down by reason of the Airs Lightness, will not do so when the Medium is thus made more heavy than themselves are.   

Why the Mercury always stands highest upon Easterly or North Easterly Winds.  
This happens by reason of the Winds blowing almost always South or South west in the Great Atlantic Ocean, on this side the 35 Degree of North Latitude; by which means it comes to pass, that when an East or North Easterly Wind blows, 'tis sure to be checked by a contrary Blast when it reaches the Ocean; whence it must needs follow that the Air will be strongly heaped up over this Island, and consequently the Mercury must stand High whensoever these Winds do blow, except they be very high and Turbulent; in which case, the Mercury may yet fall, for a Reason to be given hereafter. The following Reason may be also in some part the cause of this Height of Mercury in North and North East Winds.   

Why in great Frosts the Mercury generally stands very High.  
This proceeds partly from the foregoing Reason, but chiefly from the Coldness of those Winds that blow in Frosty Wether, which generally are North or Easterly Winds, whose coldness does strangely contract the Air, and by consequence, thicken it, as is plain in the unsealed Thermoscope, in whose Ball the Air does manifestly shrink and swell with Cold and Heat. Now when the Air in one part of the World is contracted and shrunk up into less room than before, the Air in other parts will then flow in to supply the usual height of the Atmosphere; and that being again condensed, it follows, that the Air must needs then be very weighty, and so buoy up the Quicksilver very high.   

Why Rising yet Higher in Frosty Wether, when the Quicksilver is already very High, foreshews Snow.  
The Reason is, Because Snow, when form above, is by its extreme Coldness capable of condensing the Air more powerfully than the most extreme Cold, without it can, and by consequence, makes it more thick and weighty; so that when the Quicksilver has stood high for a time, and then does yet rise more high of a sudden, we may conclude that there is Snow in the Air, which by its more intense coldness does thus suddenly contract it and more abundently increase its weight.   

Why the Mercury falls lowest in great Winds.  
The reason of this may be, For that these great Storms force out violently that Air that Stagnats in those places in which they blow, and that with so great a violence that the adjacent Air cannot come in fast enough to supply the evacuation made by so great a current of Wind: Besides which 'tis probable, That the Horizontal motion of the Wind being so violent, may take off much of the perpendicular Pressure of the Air, and that may suffer the Quicksilver to sink down so very low.   

The Reason why it Rains very seldom in great Winds, though the Quicksilver be low.  
This is because, though the Air be then too light to support almost any kind of Vapour, yet the agitation made by the Winds does prevent their falling. Brickdust or Sand in a Viol of Water, sinks when the Liquor is at rest, but when agitated by shaking, 'twill then swim so long, as that Motion continues; so here the agitation of the Air by strong Winds, keeps those Vapours from descending, which otherwise would fall, by reason of the Airs lightness, and this is the reason, why, when there is Rain in the Air, it generally holds up till these Winds cease, and then it falls down.   

Why Southern Winds sometimes produce Rain, though the Quicksilver Rise: and Northern or Eastern Fair; though the Quicksilver fall.  
To understand this, consider, That Watery Particles are rarified by the diversity of the Celestial Influence into Bodies of divers dimensions, and the more intense that Heat is that rarefies, by so much the larger will those Aquaous Particles be. This is plain in wet Linen presented before a large Fire, where you shall perceive the watery Particles to be raised from thence so gross as to be perceived by the naked Eye, whereas, had it been hung up to dry in a less intense Heat, it would have become dry by a more minute and insensible Evaporation. Now 'tis plain that what Rain comes to us from the South, does arise in parts more hot than ours, and by consequence are rarifications of the largest size, and such as are too heavy to be suspended by that weight of Air that is usually produced by Winds from that quarter.  
Whereas on the contrary, Those Vapours that are raised in colder Countries, and brought to us by Northern or Easterly Winds, are much more fine and light, and by consequence are uncapable of falling down in our Climate, though the Air should grow lighter than usual, for though this lightness may be remarkable, yet it may be still heavier than those lighter Vapours are that float in it.   

Why it should be a Sign of much Fair Wether to follow, when it reins sometime after the Quick Silver Rises before the Fair Wether comes.  
In Metrology there are undoubtedly some concurring Causes that may be very difficult for Human Wit to Fathom; all that we can do then, at most is to offer the most probable Reasons, as to this particular; know then, that as rarification of Water is the effect chiefly of the Celestial Influences and Aspects, so these Aspects are in some part the cause of suspending more strongly what they have raised by their secret Influences. Now if it rain for 2 or 3 Days, after the Quicksilver rises, before Fair Wether comes, it argues that the Celestial Influences do not concur with other Natural Causes at that Time, and by consequence, when these cease to support those Vapours that are already rarified and raised, it argues their Power to be too weak and unable to attract and rarify fresh Matter for Rain; so that when the Air is thus wholly freed from what Rain does already float in it, it will undoubtedly be followed with a large Season of Fair Wether, till the Celestial Influences do again become strong by their proper Aspects to attract fresh Matter for Foul Wether.  
Note that what has been said in this last Part, does accord only with our English Climate, and we dare not warrant them to hold good in other Countries, where the Circumstances are vastly different from this of ours.   

Some other necessary Observations.  
Dr. Wallis Observes, in Philosophical Transactions N. 10. p. 170. That his Baroscope being made with a larger Tube than Esq Boyles, did not sometimes rise so high as the Esquires by near a quarter of an Inch. I judge the Reason to be, for that the Doctor's Tube was too big in Proportion for his Cistern; and therefore for Reasons given, where I treat of the Cistern, must needs move thus Irregular.  
Dr. Beale, Philosophical Transactions, Numb. 9 p. 157. Observed a Baroscope to ascend somewhat higher in the cold Mornings and Evenings than at Midday; the Reason did undoubtedly proceed from Air gotten into the Head of the Tube, which by expanding and contracting with Heat and Cold was the Cause of this Motion,  
'Tis observed, Philosophical Transactions, Numb. 55. p. 116. That a Baroscope did usually rise in the Heat of the day, till after three or four Years standing, and then it usually sunk down with the Heat, as before it rose; the cause did undoubtedly proceed from the Quick-Silvers being not well purged from Air: and that Air in the Body of the Mercury being expanded by the Heat, did cause the Quicksilver to swell, and by consequence rise higher; whereas, when by time, it had got free into the Head of the Glass, by expanding there, the contrary effect did follow.  
If you emerge a well filled Tube into a Cistern of Mercury, and there hastily draw out the Tube perpendicularly quite out of the Quicksilver, the Mercury in the Tube will rise up to the top, with such a violence as will endanger the breaking of the Head of the Glass. This I note, as a Caution to those that have occasion to take down a Glass, lest they suffer damage by so doing, as some have done.    
FINIS.