194 



THE WEATHER 



them together as it would an empty bellows. But that is 

 prevented by coiled springs within the shells which keep them 

 more or less distended. The elasticity of the springs is practi- 

 cally constant, therefore, the gradual fluctuations in atmos- 

 pheric pressure change the compression of these sections and 

 result in a slow, up-and-down movement of the top of the set. 

 This movement is transmitted by the levers to the pen which 

 writes them on the record sheet on the revolving cylinder. In 

 most barographs, the cylinder turns once round in a week, and 

 each sheet holds a week's record. 1 



FIG. 135. The barograph. 



FIG. 136. Rain-gauge. 



217. The Measurement of Rain. Rainfall may be caught 

 in any flat-bottomed can or dish with vertical sides, placed 

 with its top exactly horizontal, out in the open, away from 

 objects that might prevent rain from falling into the gauge or 

 cause wind eddies about the gauge and interfere with a proper 

 catch. A common ruler trimmed as thin and narrow as 

 practicable so as not to raise the water level when inserted, 

 may be used to measure the depth after each storm. The gauge 

 shown in Fig. 136 is better for accurate measuring of small 



1 In addition to the pressure of the air, fully equipped weather stations 

 record its temperature, also the rainfall and snowfall, the direction and force 

 of the wind, the duration of sunshine and cloudiness, the amount of moisture 

 in the air, and a number of other conditions. Several of these are recorded 

 automatically by specially designed apparatus. 



