AIR-PUMP. 



173 



FIG. 105. 



gently there is a tendency toward the formation of a vacuum be- 

 tween the leather and the stone. The stone is 

 now pushed upward with a force of 15 Ibs. for 

 every square inch of its lower surface ( 273.) It 

 is pressed downward with a force of 15 Ibs. upon 

 each square inch of its upper surface not covered by 

 the "sucker." The downward atmospheric pres- 

 sure upon the leather is sustained by the string. 

 This difference between the upward and down- 

 ward atmospheric pressures upon the stone may be 

 greater than the gravity of the stone. Then we 

 say that the stone is pulled up by the "sucker;" 

 in reality the stone is pusJied up by the air. 



(4.) The hand-glass is a receiver open at both 

 ends. The lower end fits ac- 

 curately upon the plate of the air-pump. (It is 

 well to smear the plate with tallow in this and 

 similar experiments.) The hand is to be 

 placed over the other end. When the pump is 

 worked, the pressure of the atmosphere is felt, 

 and the hand can be removed only by a con- 

 siderable effort. The appearance of the palm 

 of the hand at the end of this experiment is due to the tension of 

 the air within the tissues of the hand. 



(5.) Repeat the experiment described in 2C9. 

 (6.) Over the upper end of a cylindrical receiver, tie tightly a wet 

 bladder, and allow it to dry. Then ex- 

 haust the air. The bladder will be forced 

 inward, bursting with a loud noise. 



(7.) Replace the bladder with a piece of 

 thin india-rubber cloth. Exhaust the air. 

 The cloth will be pressed inward and nearly 

 cover the inner surface of the receiver. 

 The hand-glass, used in experiment (4), 

 will answer for the two experiments last 

 given, by placing the small end upon the 

 pump-plate. 



(8.) Review the experiments mentioned 

 in 283. 



(9.) The" fountain in vacua" consists of 



a glass vessel through the base of which passes a tube terminating 

 in a jet within, and provided with a stop-cock and screw without. 

 By means of the screw it may be attached to the air-pump and the 



FIG. 107. 



