PNEUMATICS. 



contained any elastic vapour generated 

 during Lhe rarefaction, it would be con- 

 densed upon the re-admission of the at- 

 mospheric air, as it cannot subsist in the 

 usual pressure. The pear-guage, there- 

 fore, shows the true quantity of atmos- 

 pheric air left in the receiver. Hence it 

 will sometimes indicate that all the per- 

 manent air is exhausted from the re- 

 ceiver, except about T <^Voo Prt, 

 when the other guages do not shew a de- 

 gree of exhaustion of more than two 

 hundred times, and sometimes much 

 less. 



When the receiver is placed upon the 

 plate of the air-pump without exhausting 

 it, it may be removed again with the ut- 

 most facility, because there is a mass of 

 ail- under it, that resists, by its elasticity, 

 the pressure on the outside ; but exhaust 

 the receiver, thus removing the counter 

 pressure, and it will be held down to the 

 plate by the weight of the air upon it. 

 What the pressure of the air amounts to, 

 is exactly determined in the following 

 manner : when the surface of a fluid is 

 exposed to the air, it is pressed by the 

 weight of the atmosphere equally on 

 every part, and consequently remains at 

 rest. But if the pressure be removed from 

 any particular part, the fluid must yield 

 in that part, and be forced out of its situa- 

 tion. 



Into the receiver A, (fig. 4), put a 

 small vessel with quicksilver, or any 

 other fluk!, and through the collar of lea- 

 thers at B, suspend a glass tube, herme- 

 trically sealed, over the small vessel. 

 Having exhausted the receiver, let down 

 the tube into the quicksilver, which will 

 not rise into the tube as long as the re- 

 ceiver continues empty. But re-admit 

 the air, and the quicksilver will immedi- 

 ately ascend. The reason of this is, that 

 upon exhausting the receiver, the tube is 

 likewise emptied of air ; and therefore, 

 when it is immersed in the quicksilver, 

 and the air re-admitted into the receiver, 

 all the surface of the quicksilver is press- 

 ed upon by the air, except that portion 

 >Vhich lies above the orifice of the tube : 

 consequently, it muss rise in the tube, 

 and continue so to do, until the weight 

 of the elevated quicksilver press as forci- 

 bly on that portion which lies beneath 

 the tube, as the weight of the air does on 

 every other equal portion without the 

 tube' 



Take a common syringe of any kind, 

 and 1 \aving pushed the piston to the fur- 

 thest end, 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 HYDRAU- 

 LICS, and PUMP. 



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, 29A 

 inches high, and an inch thick, weighs 

 just 15 pounds, consequently the air prey- 



