was faften'd to the key in the cover of our receiver. This done, we con- PNEUMATICS vey'd the whole apparatus into the veffel, which being clofed up, and emptied after the ufual manner, we turn'd the key in the cover, and thereby fhortning the ftring, pull'd the trigger, and obferv'd, that the force of the fpring of the lock was not fenfibly abated by the absence of the air; for, the cock falling with its ufual violence, ftruck as many, and as confpicuous fparks of fire, as, for ought we could perceive, it did in the open air. Upon often repeating this experiment, we could not perceive, but that the fparks of fire moved upwards, downwards, and fideways, as when out of the receiver. We, likewife, fubftituted a piece of fteel for the flint, when, the piftol being cock'd, and convey'd into the receiver, we pull'd the trigger, after the air was drawn out; and tho' the place were purposely darkned, there appear'd not, upon the collifion of the two fteels, the leaft fpark of fire. We have, indeed, found, that, by the dextrous collifion of two harden'd pieces of fteel, many fparks may be struck out; but that was done with fuch a vehement percuffion of their edges, as could not well be procured in our receiver. But most of our attempts, to fire the gun-powder in the pan of the piftol, fail'd, because we were obliged to let it hang, almoft perpendicu larly, in the receiver; whereby the powder was fhook out, before the fparks could reach it. Once, however, the experiment fucceeded; and the kindled powder feem'd to make a more expanded flame, than it would have done in the open air, and mounted upwards: upon the extinction of the flame, the receiver appear'd darkned with fmoke, which feem'd to move freely up and down, and, upon letting in the air, began to circulate much fafter than before. 15. We convey'd into a small receiver, a piece of combuftible, dry, An attempt to black matter; and carefully closing the veffel, we brought it to a window, kindle a com at which the fun fhone in very freely; then, drawing out the air, we, bustible body, by the fun's with a burning-glafs, threw the fun's rays upon the combuftible matter, rays in vacuo. which began immediately to fend out a fmoke that darkned the receiver; but, notwithstanding all our care, the external air got in, and frustrated the experiment. We, therefore, lodg'd this combuftible matter in the cavity of our largest receiver, so that it was almoft contiguous to the fide next th e fun: we then endeavour'd to kindle it, but found, that by reafon of the thicknefs of the glass, the fun-beams, thrown in by the burning-glafs, were, in their paffage, fo dislocated, and scatter'd, that we could not, poffibly, unite enow of them, to make the matter yield a fenfible fmoke. 16. We convey'd into the receiver, a little pedestal of wood, in the midft An excited of which was, perpendicularly erected, a flender iron, upon the fharp needle in vacuo point whereof, an excited needle of fteel, of about five inches long, was affected by the fo placed, that, hanging in equilibrium, it could move freely every way. Then the air being pump'd out, we employ'd a load-ftone, moderately vigorous, to the outfide of the glafs, and found that it attracted, or repell'd magnet. PNEUMATICS repell'd the ends of the needle, without any remarkable difference from what the fame load-ftone would have done, had none of the air been drawn away from about the needle; which, when the load-ftone was remov'd, rested, after fome tremulus vibrations, in a position north and fouth. The Torricel 17. A flender, and very exact cylinder of glafs, near three feet in lian experi- length; its bore, a quarter of an inch in diameter; being hermetically ment in vacuo. feal'd, at one end, was, at the other, fill'd with quick-filver; care being taken, that as few bubbles as poffible, fhould be left in the mercury. Then the tube, being ftopt with the finger, and inverted, was open'd into a long, flender, cylindrical box, half fill'd with quick-filver; when that in the tube fubfiding, and a piece of paper being pafted level to its upper furface, the box and tube were, by ftrings, carefully let down into the receiver; and the cover, by means of this hole, flipt along as much of the tube, as reach'd above the top of the receiver: the interval left betwixt the fides of the hole, and thofe of the tube, being exquifitely fill'd up with melted diachylon; and the round chink, betwixt the cover and the receiver, likewife, very carefully clos'd; upon which closure, there appear'd no change in the height of the mercurial cylinder: whence the air feems to bear upon the mercury, rather by virtue of its fpring, than of its weight; fince its weight could not be fuppos'd to amount to above two or three ounces; which is inconfiderable, in comparison of fuch a cylinder of mercury as it would fuftain. Now the fucker was drawn down, and immediately, upon the evacuation of a cylinder of air, out of the receiver, the quick-filver in the tube fubfided; and notice being carefully taken of the place where it ftopt, we work'd the pump again, and mark'd how low the quick-filver fell at the second exfuction: but, continuing thus, we were foon hinder'd from accurately marking the stages in its defcent, because it prefently funk below the top of the receiver: fo that we could, from hence, only mark it by the eye. And continuing pumping, for about a quarter of an hour, we could not bring the quick-filver in the tube, totally to subside. Then we let in fome air; upon which, the mercury began to re-afcend in the tube, and continu'd mounting, till having return'd the key, it immediately rested at the height it had then attain'd. And so, by turning, and returning the key, we did, feveral times, impel it upwards, and check its afcent; till, at length, admitting as much of the external air, as would come in, the quick-filver was impell'd up, almost, to its firft height; which it could not fully regain, because some little particles of air were lodg'd among those of the quick-filver, and rose in bubbles to the top of the tube. It is remarkable, that having, two or three times, try'd this experiment, in a small veffel; upon the very firft cylinder of air that was drawn out of the receiver, the mercury fell, in the tube, 18 inches and a half; and, at another time, 19 inches and a half. We, likewise, made the experiment in a tube less than 2 feet in length; and, when there was fo much air drawn out of the receiver, that the remaining part could not counter-balance the mercurial cylinder, it fell above a span at the firft ftroke; and the external air being let in, impell'd PNEUMATICE Lastly, we obferv'd, that if more air were impell'd up, by the pump, into the receiver, after the quick-filver had regain'd its usual standard in the tube, it would afcend ftill higher; and immediately, upon letting out that air, fall again to the height it refted at before. But, in order to fill the Torricellian tube with exactnefs, the edges of the open end fhould be made even, and turned inwards, that fo the orifice, not much exceeding a quarter of an inch in diameter, may be the more eafily, and exactly stopp'd by the finger; between which, and the quick-filver, that there may be no air intercepted, it is requifite that the tube be perfectly full, that the finger, preffing upon the protuberant mercury, may rather throw fome out, than not find enough to keep out the air exactly. It is, alfo, an useful way, not quite to fill the tube, but to leave, near the top, about a quarter of an inch empty: for, if you then ftop the open end, and invert the tube, that quarter of an inch of air will afcend in a great bubble to the top; and, in its paffage, lick up all the little bubbles, and unite them with itself, into one great one. So that, if by re-inverting the tube, you let that bubble return to the open end of it, you will have a much closer mercurial cylinder than before; and need add but a very little quick-filver more, to fill up the tube exactly. And, laftly, as for fuch lefs, and invifible parcels of air, which cannot be thus gather'd up, you may endeavour, before you invert the tube, to free the quick-filver from them, by fhaking the glafs, and gently knocking on the outfide of it, after every little parcel of quick-filver pour'd in; and afterwards, forcing the bubbles to disclose themselves, and break, by applying a hot iron near the top of the glafs; which will raise the bubbles fo powerfully, as to make the mercury appear to boil. I remember, that by carefully filling a short tube, tho' not quite free from air, we have made the mercurial cylinder reach to thirty inches, and above an eighth; which is mention'd, because we have found, by experience, that in short tubes, a little air is more prejudicial to the experiment, than in long ones. curial barome ter. 18. We fill'd a glafs tube, about three feet long, with mercury; and Odd phenomehaving inverted it into a veffel of other quick-filver, that in the tube na of the merfell down to its ufual height; leaving fome little particles of air in the space it had deferted: for, by the application of hot bodies to the upper part of the tube, the quick-filver would be a little deprefs'd. Laftly, having put both the tube, and the veffel whereon it rested, into a convenient wooden frame, we placed them together in a window of my chamber. And during several weeks that the tube continu'd there, I obferv'd, that the quick-filver did, fometimes faintly imitate the liquor of a thermo meter; H PNEUMATICS meter; fubfiding a little in warm, and rifing a little in cold weather; which we afcrib'd to the greater, or leffer preffure of that little air, which remain'd at the top of the tube, expanded, or condens'd by the heat or cold of the ambient air. But the quick-filver often rofe, and fell in the tube very confiderably, after a manner, quite contrary to that of weatherglaffes, where air is at the top; for fometimes I obferv'd it, in very cold weather, to fink much lower than at other times, when the air was comparatively warmer. And fometimes the quick-filver would, for several days together, reft almost at the fame height; and at others, it would in the compass of the fame day confiderably vary its altitude; tho' there appear'd no change, either in the air abroad, or in the temper of that within my room, nor in any thing else, to which such a change could reafonably be imputed; efpecially confidering, that the space wherein the mercury continued unfettled for five weeks, amounted to full two inches ; defcending in that time about of an inch from the place where it first fettled, and afcending the other inch, and: and when we took the tube out of the frame, after it had ftaid there part of November, and December, a large fire being then in the room, we found the mercurial cylinder to be above the upper furface of the ftagnant mercury 29 inches. * That the quick-filver in the barometer Such blown to another quarter, may, not only Such an inequality in the rife, and fall of the mercury will, I fear, ren- PNEUMATICS der it difficult to determine by the barometer, whether the moon be the caufe of the tides, especially, till the reafon of this odd phenomenon be certainly known; which feems principally to depend upon confiderable alterations in the air, in point of rarity and denfity. 19. We took a tube of glafs, about four feet in length, hermetically A like experifeal'd at one end, fill'd it with common water, and inverted the open end, ment made beneath the furface of a veffel of water. Then this veffel, with the tube with water. in it, being let down into the receiver, the pump was fet on work; when, till the receiver was moderately exhaufted, the tube continu'd quite full of water; it being requifite, that a great part of the air contain'd in the receiver fhould be drawn out, to bring the remaining to an equilibrium, with fo fhort a cylinder of water. But, when once the water began to fall in the tube, each exfuction of air made it defcend a little lower; tho' nothing near fo much, nor fo unequally, as the quick-filver did. The loweft, we were able to draw down the water, was, to about a foot above the furface of that in the veffel. And, when the water was drawn down thus low, we found, that by letting in the outward air, it might be immediately impell'd up again, to the higher parts of the tube. Upon making this experiment in a small receiver, we obferv'd, that at the first exfuction of the air, the water usually fubfided feveral inches; and at the fecond, fometimes near two feet; whereupon letting in the external air, the water was impell'd up, with a very great velocity. 20. That the air hath a confiderable claftic power, we have abundantly Whether waproved; but, whether water participates, in any meafure, thereof, feems ter be elastic? hitherto, to have been fcarce confider'd. Into a large glafs bubble, with a long neck, we pour'd common water, till it reach'd about a span above the bubble; and a piece of paper being pafted thereon, we put it, unftopt, into the receiver; when, the pump transparent, fo as to afford an excellent profpect of remote objects. But, when it is heavy, and the quick-filver ftands high in the barometer, the heavens appear fair, but fomewhat thick, by reason of the vapours, every where equally difperfed therein, and is lefs fit to afford a good view of objects at a distance. And if any clouds are feen, they be very high, and move flow. When the air is at the heaviest, thick clouds fometimes cover the earth, confifting probably of fuch exhalations, as the air, at that time, is unable to fuftain; and which, cannot float therein, when it is light. In our climate, the barometer ftands highest, when the weather is coldeft, and when the ealt, or north eaft winds blow; because, at that time, two winds blow together, VOL. II. from oppofite parts; for in the Atlantic |