WEATHER INSTRUMENTS 149 



/. 1305 : 292-295. Weather and Climate. 



g. 1308 : 99-100. Climate and Factors of Climate. 



h. 1309 : 210-217. Weather and Climate. 



i. 1310 : 440-461. Climate and the Determination of its 



Zones. 



j. 1311 : 268-270. Weather and Climate. 



k. 1312 : 388-390. Weather and Climate. 



I. 1312 : 394-395. Changes in Climate Questioned. 



m. 1313 : 171-175. Weather and Climate. 



109. WEATHER INSTRUMENTS 



It is desirable, for many reasons, to measure the factors 

 which go to make the weather, such as rainfall, temperature, 

 wind velocity and direction, humidity, atmospheric pressure, 

 and sunshine. The different pieces of apparatus used for this 

 purpose are called weather instruments. 



The rain gauge collects the water which falls in a measured 

 area, and the thickness of the layer of rain which fell is easily 

 computed, or even measured. 



Temperature is measured by a metallic thermometer which 

 records its readings on a moving cylinder of paper. Its name 

 is thermograph. The recording barometer makes its record 

 in much the same way, and is called a barograph. 



Wind direction is indicated by a weather vane; its velocity 

 by revolving hemispheres, called an anemometer. 



Humidity is measured by the dew-point apparatus or by 

 means of a wet-bulb thermometer. The drier the air, the 

 greater the evaporation from the cloth around the wet bulb, 

 and therefore the temperature of the bulb is reduced. Tables 

 of relative humidity may be consulted for desired answers. 



The sunshine recorder absorbs enough heat from the sun- 

 shine to close an electric circuit by means of expanding mer- 



