HEIGHT AND "WEIGHT OF THE ATMOSPHERE. 



215 



A glass jar with a 

 mouth six inches 

 across would need 

 a force equal to 

 nearly four hund- 

 red jDounds to dis- 

 place it. If there 

 he a glass vessel 

 open at both ends, 

 the hand placed on 

 the top may he so 

 firmly held by the 

 pressure that it can not be removed Fig. 179. 



until the air is again admitted below 

 {Fig. 179). If a thin plate of glass 

 be placed on the top of this open ves- 

 sel, on pumping out the air, the 

 weight will suddenly crush it with a 

 noise like the report of a gun. 



Some interesting instances occur in nature of the 

 use of atmospheric pressure. Flies walk on glass by 

 means of the pressure against the outside of their feet, 

 the air having been forced out beneath. In a similar 

 way, some kinds of fishes cling to the sides of rocks 

 under water, so as not to be swept off" by the current. 

 Dr. Shaw threw a fish of this kind into a pail of water, 

 and it fixed itself so firmly to the bottom, that, by tak- 

 ing hold of the tail, he lifted iip the pail, water and all. 

 It is the pressure of the atmosphere upon water that 

 drives it up the barrel of a pump as soon as the air is 

 pumped out from the inside. Hence the reason that 

 pumps can never be made to draw water more than 



The Hand fastmed 

 by Air. 



