PNEUMATICS. 



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 5,575,080,000, 

 000,000 square feet on the Earth's sur- 

 face ; which number, multiplied by 2,160 

 pounds (the pressure on each square 

 foot), 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, but 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 ti.e air striking forcibly 

 against the inside of the receiver, upon 

 bt -ing re-admitted. Air, as we have seen, 

 is one of the most elastic bodies in nature ; 

 that js, 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 tiie 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 

 thav 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 dis- 

 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 nuich 

 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 of such 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. 



The elastic power of the air is always 

 equivalent to the force which compresses 

 it ; for if it were less, it would yield to 

 the pressure, and be more compressed; 

 were it greater, it would not be so much 

 reduced ; for action and re-action are al- 

 ways equal, so that the elastic force of 

 any small portion of the air we breathe, 

 is equal to the weight of the incumbent 

 part of the atmosphere ; that weight be- 

 ing the force which confines it to the di- 

 mensions it possesses. 



To prove this by an experiment, pour 

 some quicksilver into the small bottle, A, 

 (fig. 7), and screw the brass collar, C, of 

 the tube, BC, into the brass neck of the 

 bottle, and the lower end of the tube will 

 be immersed into the quicksilver, so that 

 the air above the quicksilver in the bottle 

 will be confined there. This tube is open 

 at top, and is covered by the receiver, 

 G, and large tube, EF ; which tube is 

 fixed by brass collars to the receiver, and 

 is closed at top. This preparation being 

 made, exhaust the air out of the receiver, 

 G, and its tube, by putting it upon the 

 plate of the air pump, and the air will, 

 by the same means, be exhausted out of 

 the inner tube, BC, through its open top 

 at C As the receiver and tubes are ex- 

 hausting, the air that is confined in the 

 glass bottle, A, will press so by its spring, 

 as to raise the quicksilver in the inner 

 tube to the same height as it stands in 

 the barometer. 



There is a little machine, consisting of 

 two vanes of equal weights, independ. 

 ent of each other, and turn equally free 

 on their axles in the frame. Each vane 



