Take a common syringe of any kind, and having end, the piston to the fur thest en, immerse it into water ; then draw up the piston, and the water will follow it. This is owing to the same cause as the last: when the piston is pulled up, the air is drawn out of the syringe with it, and the pressure of the atmosphere is removed from the part of the water im mediately under it ; consequently, the water is obliged to yield in that part to the pressure on the surface. It is upon this principle that all those pumps called sucking pumps act : the piston fitting tightly the inside of the barrel, by being raised up, removes the pressure of the atmosphere from that part, and conse quently the water is drawn up by the pressure upon the surface. See HTunAC airs, and Prmr.
The effects arising from the weight and pressure of the atmosphere have been ab surdly attributed to suction ; a word which ought to be exploded, as it conveys a false notion of the cause of these and similar phenomena. To prove that an ex hausted receiver is held down by the pressure of the atmosphere, take one, open at top, and ground quite flat, as A, (fig. 6), and covered with a brass plate, B, which has a brass rod passing through it, working in a collar of leather, so as to be air tight ; to this rod suspend a small receiver within the large one, a little way from the bottom ; place the receiver, A, upon the pump-plate, and exhaust it : it will now be fixed fast down ; but the small receiver may be pulled up or down with perfect ease, as it is itself exhausted, and all the air which surrounded it removed, consequently it cannot be exposed to any pressure ; let, then, the small one down upon the plate, but not over the hole by which the air is extracted, and re-admit the air into the large receiver, which may then be removed; it will be found, that the small one being itself exhausted, is held down fast by the air, which is now admitted round the outside. If the large receiver be again put over it and exhaust ed, the small one will be at liberty, and so on, as often as the experiment is re peated. This effect cannot be accounted for upon any other principle than the pressure of the air ; as the common idea of suction can have nothing to do in the case of the small receiver, which is fixed down merely by letting in the air round it. We ought, therefore, to attribute all those effects which are vulgarly ascribed to suction, such as the raising of water by pumps, &c. to the weight and pressure of the atmosphere.
A square column of quicksilver, inches high, and an inch thick, weighs just 15 pounds, consequently the air press es with a weight equal to 15 pounds, up on every square inch of the Earth's sur face ; and 144 times as much, or 2,160 pounds, upon every square foot. The Earth's surface contains in round num bers, 200,000,000 square miles ; and as every square mile contains 27,876,400 square feet, there must be 5,575,030,000, 000,000 square feet on the Earth's sur face ; which number, multiplied by 2,160 pounds (the pressure on each square fix)°, gives 12,043,468,800,000,000,000 pounds for the pressure, or whole weight of the atmosphere If the top of a small receiver be cover ed by a piece of flat, thin glass, upon exhausting it, the glass will be broke to pieces by the incumbent weight ; and this would happen to the large receiver itself, hut for the arched top, that resists the weight much more than a flat surface.
This experiment may be varied, by ty ing a piece of wet bladder over the open mouth of the receiver, and leaving it to dry till it becomes as tight as a drum. Upon exhausting the receiver, you will perceive the bladder rendered concave, and it will yield more and more, until it break with a loud report, which is oc casioned by tim air striking forcibly against the inside of the receiver, upon bring re-admitted. Air, as we have seen, is one of the most elastic bodies in nature ; that is, it is easily compressed into less compass, and when the pressure is re moved it immediately regains its former bulk As all the parts of the atmosphere gravitate, or press upon each other, it is easy to conceive, that the air next the sur face of the earth is more compressed and denser than what is at some height above • it ; in the same manner as if wool were thrown into a deep pit until it reached the top. The wool at the bottom having all the weight of what was above it, would be squeezed into a less compass ; the layer, or stratum above it, would not be pressed quite so much ; the one above that still less, and so on, till the upper one, having no weight over it, would be in its natural state. This is the case with the air, or atmosphere, that surrounds our earth, and accompanies it in its mo tion round the sun. On the tops of lofty buildings, but still more on those of moun tains, the air is found to be considerably less dense than at the level of the sea. The height of the atmosphere has never yet been exactly ascertained ; indeed, on account of its great elasticity, it may ex tend to an immense distance, becoming, however, rarer, in proportion to its die tance from the earth. It is observed, that at a greater height than forty five miles it does not refract the rays of light from the sun ; and this is usually consider ed as the limit of the atmosphere. In a rarer state, however, it may extend much further. And this is by some thought to be the case, from the appearance of cer tain meteors which have been reckoned to be seventy or eighty miles distant, and whose light is thought to depend upon their coming through our atmos phere. Dr. Cotes has demonstrated, that if altitudes in the air be taken in arith metical proportion, the rarity of the air will be in geometrical proportion. And hence it is easy to prove by calculation, that a cubic inch ofsuch air as we breathe, would be so much rarefied at the alti tude of .500 miles, that it would fill a sphere equal in diameter to the orbit of Saturn.