PRESSURE OF THE ATMOSPHERE. 89 



against the walls of the vessel enclosing it is called its elas- 

 tic force. 



The action of a common syringe will serve to illustrate 

 the elasticity of atmospheric air. If the piston be drawn 

 out, and the open end of the syringe then closed, a consid- 

 erable effort will be required to force in the piston to more 

 than a small part of the length of its range, and if the 

 syringe be air-tight and strong enough, it will require the 

 application of great power to force the piston down through 

 nearly the whole of its range. This experiment also shows 

 that the pressure increases with the compression, the air 

 within the syringe acting as an elastic cushion. If the 

 piston be let go, after being forced in, it will be driven 

 back, the air within expanding to its original volume. 



An inverted glass cylinder, carefully immersed in water, 

 furnishes another simple illustration of the elasticity of air. 

 Holding the cylinder vertical, it may be 

 pressed down in the water without much 

 loss of air, and it will be seen that the sur- 

 face of the water within the vessel CD is 

 below the surface of the water outside AB. 

 It is evident that the downward pressure of 

 the air within at CD is equal to the upward ^^ 

 pressure of the water at the same place, F'g-30 



which (Art. 11, Cor. 2) is equal to the pressure on the up- 

 per surface AB, increased by the pressure due to the depth 

 of the surface CD below the upper surface ; hence the air 

 within, which has a diminished volume, has an increased 

 pressure. 



41. Pressure of the Atmosphere. If a glass tube* 

 about three feet in length, closed at one end, be filled with 

 mercury, and then, with the finger pressed to the open end 



* This experiment was first made by Tom'celli, and hence IB called TorrietUVs 

 Experiment, and the vacant space above the mercury in the tube is called the Torri- 

 cellian Vacuum. 



