THE AIR 155 



watch and are convenient to carry. The number of feet 

 above sea level is marked on the dial, so that one can 

 determine the elevation simply by looking for the num- 

 ber to which the hand points. These, however, must 

 be set with the mercury barometer before starting on a 

 trip to measure elevation; or if the reading is taken at 

 the base of a hill and then at its top, the height of the 

 hill above the plain can be determined, but not its height 

 above sea level. 



The barograph is a form of the aneroid barometer. 

 Instead of having a hand moving around a dial to indi- 

 cate the pressure, the barograph 

 has an arm with a pen at the 

 end, which writes the air pres- 

 sure on a sheet of paper pre- 

 pared for it. This sheet of paper 

 has the hours of the day and the 

 inches printed on it, and is A BAROGRAPH 



fastened on a roller which is turned by clockwork. As 

 the hours pass the roller turns and an ink line is made on 

 the paper, indicating the air pressure. 



104. Air Currents. The atmosphere is constantly 

 in motion, moving in various directions. When the air 

 is rising in one locality, it is descending in another. 

 When the air near the surface is moving north, a current 

 above it is moving south. Currents above each other 

 are usually moving in opposite directions. The two 

 principal causes of the air movements are: (i) The 

 unequal heating of the air by the earth's surface and (2) 

 the varying amount of moisture or water vapor in the air. 

 Either one of these causes affects the air pressure and 

 hence affects the barometric reading. The wind blows 

 toward the place where the barometer reads low and 



