PNEUMATICS. 131 



What the pressure of the air amounts to is ex- 

 actly 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 situation. 



Into the receiver A (Plate 6. fig. 2.), put a 

 small vessel with quicksilver, or any other fluid, 

 and through the collar of leathers at B, suspend a 

 glass tube, hermetically sealed, (that is, closed by 

 the glass blower,) 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 receiver continues empty. But re- 

 admit the air, and the quicksilver will immediately 

 ascend. The reason of this is, that upon ex- 

 hausting 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 re- 

 ceiver, all the surface of the quicksilver is pressed 

 upon by the air, except that portion which lies 

 above the orifice of the tube; consequently, the 

 quicksilver must rise in the tube, and continue so to 

 do, until the weight of the elevated quicksilver press 

 as forcibly 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 having 

 pushed the piston to the farthest 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 



K 2 



