WEIGHT OF THE ATMOSPHERE. 241 



be no larger than a small tumbler. A glass jar with a 

 mouth six inches across would need a force equal to nearly 

 four hundred pounds to displace it. If Fig. 264. 



there be a glass vessel open at both ends, 

 the hand placed on the top may be so 

 firmly held by the pressure that it can not 

 be removed until the air is again admit- 

 ted below (fig. 264). If a thin plate of 

 glass be placed on the top of this open The H^^^dJ^tened 

 vessel, on pumping out the air, the ^y -^^r. 



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 taking hold of the tail, he 

 lifted up 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. Plence the reason that 

 pumps can never be made to draw water more than thirty- 

 three fe^ below the piston, a height coiTesponding to the 

 weight of the atmosphere. In practice they never draw 

 water even to this height, as a perfect vacuum can not be 

 made by pumping. 



THE BAROMETER. 



On the same principle the Barometer is made. It con- 

 sists of a glass tube, nearly three feet long, open at one 

 end, and which is first filled with mercury, a liquid nearly 

 fourteen times heavier than water. The open end is then 

 U 



