PrZUMATICS before the mouth of it was open'd, under the ftagnant mercury, and notice taken how low fuch a quantity of that air deprefs'd the mercurial cylinder, 'twas obferv'd, that at the mountain's foot, the included air was not able to deprefs the quick-filver fo much. Whence we infer, that the cylinder of air, at the top of the hill, being fhorter and lighter, did not fo ftrongly prefs against the included air, as did the ambient air at the bottom of the hill, where the aerial cylinder was longer, and heavier.

Fig. 83.

We, alfo, attempted a trial, wherein we hoped to find a fenfible difference in the weight of the atmosphere, in a far lefs height, than that of an ordinary hill. But instead of a common tube, we made ufe of a weather-glass, and inftead of quick-filver, employ'd common water in the pipe belonging to the glass; that fmall changes in the weight or refiftance of the atmofphere, in oppofition to the included air, might be the more difcernible.

The inftrument, we made ufe of confifted only of a glass AB, with a broad foot, a narrow neck, and a flender glafs pipe CD, open at both ends; the pipe fo plac'd, that the bottom ofit, almoft reach'd to the bottom of the bigger glafs AB, within whofe neck A, it was faften'd with a clofe cement, that both kept the pipe in its place, and hinder'd all communication betwixt the inward II, and the outward air KK, except by the cavity of the pipe CD. Now we chofe this glafs AB, more than ordinarily capacious, that the effect of the dilatation of the included air II, might be the more confpicuous. Then conveying a convenient quantity of water HD, into this glafs, we carry'd it to the leads of the abbey-church at Westminster, and there blew in a little air, to raise the water to the upper part of the pipe, that, being above the veffel AB, we might the more precifely mark the feveral ftations of the water. Afterward, having fuffer'd the glass to reft a pretty while upon the leads, that the air II, within, might be reduced to the same state with KK, that without; having mark'd the ftation of the water F, we gently let down the veffel by a string to the foot of the wall, where one attended to receive it, who having fuffer'd it to rest upon the ground, told us, that it was fubfided about an inch below the mark F; whereupon, having order'd him to put a mark at this fecond ftation of it E; we drew up the veffel again, and fuffering it to reft a while, obferv'd the water to be re-afcended to the firft mark F,which was, indeed, about an inch above E: and this we did a fecond time, with almost a like fuccefs; tho', two or three days after, the wind blowing ftrongly upon the leads, we found not the experiment to fucceed quite fo regularly; yet the water, always, manifeftly, fell lower at the foot of the wall, than at the top. But, to avoid mistakes, and prevent objections, we made the experiment within the church, at the fame height with the leads; but the upper part of the pipe being, accidentally, broken off, we order'd the matter fo, that the furface G, of the remaining water in the pipe, fhould be about an inch higher than the furface of that in the veffel. And then,. letting down the glafs, I found that, almoft, as foon as it was fettled upon the the pavement, it was not only fallen as low as the other water, but the outward air deprefs'd it fo far, as, whilft I was looking on

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གྲ་དུ་

to break in below the bottom of the pipe, and afcend thro' the water in PNEUMATICS. bubbles; after which, the glafs being drawn up again, the water was, very manifeftly, re-afcended. Hence 'tis evident, that the atmosphere gravitates more, cateris paribus, near the furface of the earth, than in the more elevated parts of the air for the leads, on which we made our trials, were found, in perpendicular height, but 75 feet.

But, for an experiment of the fame kind, made at a greater height, take the following, communicated by Dr. Power.

On the 15th of October, 1661, we took a weather-glafs AB, about Fig. 84. two feet in length, and carrying it to the bottom of Hallifax-hill, the water flood in the fhank at 13 inches above that in the veffel: thence carrying it, thus fill'd, with the whole frame, immediately to the top of the faid hill, the water fell down to the point D; that is, an inch and a quarter lower than it was at the bottom of the faid hill; which proves the elafticity of the air: for the internal air AC, which was of the fame power and extenfion with the external, at the bottom of the hill, manifefted a greater elafticity, than the mountain-air there manifefted preffure; and fo" extended itself further by CD.

The like experiment, I hear, the fame ingenious perfon has lately repeated, and found the defcent of the water to be greater than before. And tho' fome have thought it ftrange, that, on a hill, far inferiour to the Alps, and Appennines, fo fhort a cylinder of water fhould fall fo much; yet I fee not any reafon to diftruft, upon this ground, either this experiment, or ours made at Westminster; but rather wonder the water fell no more, if the hill be confiderably high: for their fufpicion feems grounded on a mistake; as if because the quick-filver, in the Torricellian experiment, made without purpofely leaving any air in the tube, would not at the top of the mention'd hill, have fubfided above an inch, the water, that is near 14 times lighter, fhould not fall above a 14th part of that fpace; whereas, in the Torricellian experiment, the upper-part of the tube has little, or no air left in it, while the correfpondent part of the weather-glafs contain'd air, whofe preffure was little lefs than that of the atmosphere at the bottom of the hill; and, confequently, must be much greater, than the preffure of the atmosphere at the top of the hill..

Another particular, which confirms our hypothefis, is that experiment made by the fame M. Pafchal, by carrying a flack-blown foot-ball, from the bottom to the top of an high mountain; for, the foot-ball gradually fwell'd, the higher it was carry'd fo that at the top of the mountain it appear'd as if it were full-blown; and became gradually lank again, as it was carry'd downwards; fo that, at the foot of the hill, it was faccid as before. We have here an experiment to prove our hypothesis, wherein recourfe cannot be had to any body, forcibly, and preternaturally diftended, fuch as is pretended to remain in the deferted space of. the tube, in the Torricellian experiment.

But, further, our author's hypothefis is needlefs; for, he denies not that the air has fome weight and fpring, but affirms it very infufficient

to.

PNEUMATICS to counterpoife a mercurial cylinder of 29 inches. We fhall, therefore, now endeavour to manifeft by experiments, purpofely made, that the spring of the air is capable of performing far more than is neceffary to folve the phenomena of the Torricellian experiment. We took a long glafs tube, prefs'd and di- fo bent at the bottom, that the part turned up, was almoft parallel to the

The elastic force of com

lated air,

measured.;

Fig. es.

reft of the tube; and the orifice of this fhorter leg being hermetically feal'd, the length of it was divided into inches, each of which was fub-divided into eighths, by a lift of paper carefully pafted along it: then putting in as much quick-filver as fill'd the bended part of the fiphon, that the mercury ftanding in a level, might reach, in the one leg, to the bottom of the divided paper, and juft to the fame height, in the other; we took care, by frequently inclining the tube, that the air, at laft, included in the fhorter cylinder, fhould be of the fame laxity with the reft of the air about it. This done, we began to pour quick-filver into the longer leg of the fiphon; which, by its weight, preffing upon that in the fhorter, gradually ftraitned the included air; and continuing to pour in quickfilver, till the air, in the shorter leg, was, by condenfation, reduced to take up but half the fpace it poffefs'd before, we obferv'd, in the longer leg of the glafs, on which was, likewife, pafted a lift of paper, divided into inches, and parts, that the quick-filver was 29 inches higher than in the other. Hence we fee, that as, according to our hypothefis, the air, in that degree of denfity, and correfpondent measure of refiftance, whereto the weight of the incumbent atmosphere reduces it, is able to balance, and refift the preffure of a mercurial cylinder of about 29 inches fo, here, the fame air, brought to a degree of denfity, about twice as great as it had before, obtains a fpring twice as ftrong; being able to fuftain, or refift a cylinder of 29 inches, in the longer tube, together with the weight of the atmospherical cylinder, that refted upon thofe 29 inches of mer

cury.

After fome other trials, one of which we made in a tube, whofe longer leg was perpendicular; and the other, that contain'd the air parallel to the horizon; we, at laft, procured a tube, which, tho' large, was fo long, that the cylinder, whereof the fhorter leg of it confifted, admitted a lift of paper divided into 12 inches, and their quarters; and the longer leg another, feveral feet in length, and divided after the fame manner: then quick-filver being poured in, to fill up the bended part of the glafs, that the furface of it, in either leg, might reft in the fame horizontal line; more quick-filver was pour'd into the longer tube : and notice being taken, how far the mercury rose therein, when it appear'd to have afcended to any of the divifions in the fhorter; the several obfervations that were thus fucceffively made, and fet down, afforded us the following table.

A

For the better underftanding of this experiment, it is proper to obferve the following particulars. 1. The tube being very tall, we were obliged to use it on a pair of ftairs, which were very well illumined; and for prefervation, it was fufpended by ftrings. 2. The lower, and bent part of the pipe, was placed in a fquare wooden box, large and deep, to prevent the lofs of the quick-filver. 3. We were two, to make the obfervation_together; the one to take notice at the bottom, how the quick-filver rose in the fhorter cylinder; and the other, to pour it in at the top of the longer. 4. The quick-filver was pour'd in but flowly, according to the direction of him who obferv'd below. 5: At the beginning of the operation, that we might the more truly difcern where the quick-filver refted, from time to time, we made ufe of a fmall looking-glafs, held in a convenient posture, to reflect to the eye what we delired to fee. 6. When the air was crowded into lefs than a quarter of the space it poffeffed before, we try'd whether the cold of a linen-cloth, dipp'd in water, would condenfe it and it, fometimes, feem'd a little to fhrink, but not fo manifeftly, that we dare build upon it. We then try'd, likewife, whether heat would dilate it; and, approaching the flame of a candle to that part where the air was pent up, it had a more fenfible operation than the cold before; fo that we fcarce doubted the expanfion of the air would, notwithstanding the weight that opprefs'd it, have been made confpicuous, if the fear of breaking the glafs had not kept us from increafing the heat..

This

PNEUMATICS This fufficiently proves the principal thing for which I here alledge it;

fince 'tis evident, that as common air, when reduced to half its natural
extent, obtain'd a fpring, about twice as forcible as it had before; fo the
air, thus comprefs'd, being farther crowded into half this narrow room,
thereby obtain'd a fpring as ftrong again as that it laft had, and confe-
quently, four times as ftrong as that of common air. And, there is no
caufe to doubt, that if we had been furnish'd with a greater quantity
of quick-filver, and a very trong tube, we might, by a further com-
preffion of the included air, have made it balance the pressure of a far tal-
ler, and heavier cylinder of mercury. For no man, perhaps, yet knows,
how near to an infinite compreffure the air may be reduced, by a force com-
petently increas'd.
So that, here our author may plainly fee, the
fpring of the air can refift, not only the weight of twenty-nine inches, but,
in fome cafes, above one hundred inches of quick-filver; and this, without
the affiftance of his Funiculus, which, in our prefent cafe, has no pretence
to be employ'd. And, to fhew, that the weight of the incumbent atmo-
fphere, made a part of the weight refifted by the imprison'd air; when the
, mercurial cylinder, in the longer leg of the pipe, was about one hundred
inches high, we caus'd a man to fuck at the open orifice, whereupon the
mercury in the tube confiderably afcended: which phenomenon cannot be
afcrib'd to our author's Funiculus; fince, by his own confeffion, that can-
not pull up a mercurial cylinder of above twenty-nine or thirty inches.
And, therefore, the preffure of the atmofphere, being in part taken off,
by expanding itself into the man's dilated cheft, the imprifon'd air, was,
thereby enabled, manifeftly, to dilate, and repel the mercury that com-
prefs'd it, till there was an equality of force betwixt the ftrong fpring of
the comprefs'd air on the one part, and the tall-mercurial cylinder, with the
contiguous dilated air, on the other.

Now, if to what we have deliver'd concerning the compreffure of the air, we add fome obfervations of its fpontaneous expanfion, it will the better appear, how much the phenomena of thefe mercurial experiments depend upon the different measures of ftrength to be met with in the air's fpring, according to its various degrees of compreffion and laxity.

A