LECTURE X.

LADIES AND GENTLEMEN,

WHE

HEN we examine clofely thofe articles, whofe apparent fimplicity has determined mankind to confider them as elements, we find them little understood, and much remaining to be developed: they may feem fimple, uniform, homogeneous; but this rather arifes from our inability to purfue their properties through their various minuter fubdivifions and branches, than from an actual elementary termination of nature's principles. Of this, our prefent fubject, the AIR, is an inftance; which, though it has ufually been thought pure and uncompounded, yet is found to include many and confiderable varieties; it poffeffes alfo many remarkable peculiarities, whofe effects are among the most striking and amufing.

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It would have been thought formerly a whimsical, if not a ftupid opinion, that the air is heavy, yet fuch is the fact; and were it fo applied, "a man might as well be prefied to death by a weight of air, as by any other means. It is also compreffible into a fmaller compafs or space, than it naturally occupies; and, on the contrary, is expanfible into a larger compass or space than ufual; from which principles proceeds its different denfity, at different heights above

the

the furface of the earth. The air is a fluid, in fome instances pretty much refembling water; but totally unlike it, in that it cannot be congealed, or rendered folid, or brought towards folidity. We can by no means exhibit a lump of air; yet we know that air enters into the compo fition of all folid bodies, and perhaps I may even fay, they owe their folidity to it. The oak is greatly indebted to air for its hardnefs; metals and stones are partly formed of air, and every fubftance of every nature includes this as one ingredient.

The reafonings of mankind proceed always by comparison: earth and water are heavy; this our fenfes inform us, and none ever carried either without acknowledging the fact but as mankind had no occafion to carry air, their fenfes were incompetent to inform them of its weight; nor indeed was it capable of being weighed, till fome contrivance was difcovered, whereby a veffel emptied of air might fhew a difference from its weight when full. Of late years this has been carried to perfection; and upon extracting the air from any proper fhaped veffel, and weighing it before and after fuch extraction, by the difference of weights we find, that a cubic foot of air weighs upwards of one ounce.

From the foregoing premifes it appears, that the earth is every where furrounded by a ponderous fluid, rifing very high above us at the bottom of this fluid (especially), are seen myriads of creatures, whofe exiftence depends upon it, who are perpetually moving about in it, and who, though compreffed by it on all fides, are fo adapted to their fituation, as to feel no inconvenience from this immerfion. Not only fo,

but

but change their fituation, they will quickly complain of insupportable anguish, and only by reftoring this indifpenfable fluid, can comfort and enjoyment be reftored to the sufferers. Nor is this all, we faid, " at the bottom of this fluid" is the station of these beings, and to this only they are adapted; for the air becoming thinner in proportion to its height, it becomes proportionally unfit for refpiration, the functions of life are performed with difficulty, and every exertion is accompanied by pain, weakness, and laffitude: on the contrary, I doubt not, that could we penetrate far below the fuperficies of our habitation, we should find equal pain and equal imbecility, from the too great denfity of that medium which should contribute to the support of life. We cannot breathe in a fluid fo dense as water; but defcending below the surface, the air is capable of much further compreffion and density than water.

It is indeed true, that we are happily fo conftructed, as not to be exceffively affected by the fmaller variations of air to which we are expofed; yet if thefe, as is undeniable, have their influence upon us, we may well conceive that by extreme variations we should be rendered uneafy and restless, if not miferable.

The general WEIGHT of the air, is proved, (1) by weighing it as I have afferted, in a very nice balance; which is performed by placing in one fcale a thin glafs flafk, whofe capacity is known, exhausted of its air (under the Air-pump); counterpoise this by grain weights, then admitting the air into the flask, it will gradually overweigh its counterpoife. The weight of the air is alfo proved, (2) by exhaufting the air from a glafs goblet (which is easily done by burning a bit of paper in it),

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then

then quickly turning it mouth downward on water, the water will rife, and fill the inverted glass; and if we attend to the reason of this, we shall find it to be, because the weight of the air, on the furface of that part of the water not covered by the glafs, preffes the water beneath it, and forces it to wherever it may find admiffion: i. e. in this cafe under the glass.

There must however, it is evident, be fome bounds to the weight of the air; I fay if we fuppofe fuch a glafs heightened, and the water rifing, there must be fome point at which it will ftop, at which the weight of the preffing air, will be counterpoifed by the weight of the admitted water; and this is found to be at the height of thirty-two feet of water. In clear words, the weight of thirty-two feet of water is equal to the weight of an equal column of the whole atmosphere; whofe height we know to be very confiderable. This has been experimented by pipes purposely made to this length. The ratio of the weight of air to water being known, we quickly find its ratio to any denfer fluid. Thus mercury being fourteen times heavier than water, a column of air will be able to fuftain a column of mercury only one fourteenth part the height of the column of water; that is to fay, about 28 or 29 inches. On this principle barometers are conftructed; and by this inftrument we learn, that though the air be always heavy, and nearly the fame, yet its weight varies, being fometimes a little. more, fometimes a little lefs. By long obfervation it is found, that the column of mercury does not ufually rife higher than 31 inches, nor fall lower than 28 inches, the medium is therefore 29 inches and a half.

No. IX.

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This

This motion, or rifing and falling of the mercury, indicates the more or less vaporated state of the atmosphere; for vapours being lighter than air, rife and float in it, and by occupying more fpace than an equal quantity of air, they diminish the weight of that part where they abound; confequently, a fhorter column of mercury may counterpoise it. But when the air is free from vapours, its native weight requires a taller column to equal its preffure.

We have faid, a column of mercury of fuch an height will counterpoife a column of the whole atmosphere of equal dimenfions;-now, if we afcend to any confiderable height, we may be faid to cut off a portion of the column of atmosphere (and that the densest part of it), equal to the height we afcend; it follows, that a proportionate part of the column of mercury may be fpared, and we actually find, that the mercury will fubfide nearly one-tenth part of an inch for every hundred feet of perpendicular height; and hereby this inftrument becomes a valuable method of measuring the heights of mountains, &c. merely by its indicating the proportionate weight of the air on their tops.

The most wonderful application of the air's weight, is in the engine for railing water from great depths, mines, &c. commonly called the fire-engine-we know that human ftrength fails and needs recruit, but the bufinefs of this engine fuffers no ceffation; if we change human strength for that of animals, they are incompetent to the business: but the philofopher, by means of a little steam to procure a vacuum in a cylinder, a little cold water to condense the steam, and the weight of the air upon the top of the cylinder, will

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produce