THE ATMOSPHERE. 



COMMON PUMP. 



A pump may be considered as a simple cylinder, 

 with a box or valve fixed at one end, and another, mov- 

 able by a piston, connected with a handle at the upper 

 end of the cylinder. The first elevation of the handle 

 causes the upper valve to descend, and when it is de- 

 pressed, the valve is made to rise. The result of 

 this action, when the pump is prepared for uso, is to 

 lift the air on the first depression of the handle, and 

 cause a partial vacancy between the lower box and the 

 one that has been raised. The air not being allowed 

 to enter from above through the valve, is removed from 

 the place it had occupied, while that resting on the sur- 

 face of the water, surrounding the pump, forces it up to 

 supply the deficiency. The piston being again de- 

 pressed, meets with the water that has passed through 

 the lower valve, which readily lifts the upper box, and 

 passing through it, is easily raised when the handle is 

 again pressed down. A continuance of this action soon 

 produces a regular stream from the spout of the pump. 



An effect precisely similar to this, is produced in the 

 elevation and depression of the piston of the airpump. 

 The air passes through its valves in the same manner 

 as the air and water have passed through those of the 

 common pump. The barrel of the airpump is connect- 

 ed by a tube to a flat plate of glass or metal, on which the 

 glass or receiver, as it is called, has been placed to 

 have its air taken away. The principle of the airpump 

 depends on the repulsion or elasticity of the air in ex- 

 panding by the removal of pressure. At each eleva- 

 tion of the piston a portion of air escapes, whose place 

 is immediately occupied by the expansion of that which 

 remains. This expansion continues until the repulsive 

 powers of the air are no longer able to expel those 

 particles which rest at the bottom of the piston. 



The result of the removal of air from a vessel is 

 called a vacuum. This must be produced in all vessels 

 intended to exhibit the pressure of the air. It exists in 

 the barometer between the fluid and the top of the tube, 

 and is called the Torricellian vacuum, which is the most 

 perfect that art can produce. 



