130 ELEMENTS OF GENERAL SCIENCE 



air proceeds, the barometer falls until, if the air pump is a 

 good one, it may show a height of only a few millimeters or 

 less. If the atmosphere is readmitted, the barometer rises to 

 its usual height. 



146. Pressure in other fluids. The laws of pressure hold 

 good for any liquid, allowance being made in the calculations 

 for the difference in weight of unit volumes of different liquids. 

 The same laws are true of gases, excepting that it is always 

 necessary to take into consideration the fact that the weight 

 of a given volume of a gas varies with the temperature and 

 the pressure, and that the lower layers of a gas (for example, 

 the lower parts of the earth's atmosphere) are more highly 

 compressed and therefore weigh more to a given volume than 

 the higher layers do. 



147. Dams. The building of dams in order to confine water 

 or raise the surface to a higher level is very common (fig. 69). 

 Dams need to be constructed with a great deal of care in order 

 to be able to withstand the pressures to which they are sub- 

 jected. A great many lives and much property have been 

 lost by reason of the floods which have been released by 

 breaking dams. One of the greatest catastrophes of this kind 

 was the breaking of the dam near Johnstown, Pennsylvania, 

 in 1889. About two thousand lives were lost in the flood 

 which resulted. 



The pressure to which a dam will be subjected may be easily 

 ascertained. The process is the same as that used in finding 

 the pressure against one of the walls of the aquarium tank. It 

 depends only upon the depth of the water and the area of the 

 submerged part of the dam. From the principle that the pres- 

 sure is independent of the size and shape of the containing 

 vessel, it follows that the area of the body of water held back 

 by the dam has nothing to do with the matter. The dikes which 

 protect Holland from the ocean need be no stronger than those 

 which protect the lowlands of the Mississippi River, even though 

 the former are holding back the whole Atlantic Ocean. 



