124 



Our Surroundings 



One form of the hygrometer consists of a dry bulb ther- 

 mometer and a wet bulb thermometer. The wet bulb is wrapped 

 with a piece of loosely woven cloth attached to a lamp wick, the 

 lower end of which rests in a cup of water. 



RELATIVE HUMIDITY TABLE FOR USE WITH A HYGROMETER 



Experiment to Determine the Amount of Water (Rela- 

 tive Humidity) in the Air by the Use of a Hygrometer. 



Expose the hygrometer to the air to be tested. Notice the tem- 

 perature on the dry bulb ther- 

 mometer. Then notice the num- 

 ber of degrees lower the wet 

 bulb thermometer reads. Then 

 turn to the relative humidity 

 table. Follow across the table 

 on the line giving the reading 

 of the dry bulb thermometer 

 to the column which indicates 

 the number of degrees lower 



on the wet bulb reading. The 



A SIMPLE RAIN GAUGE 



If the water in the bottle were spread over 



an area the size of the top of the funnel, 



it would give the depth of the rainfall. 



figure at this intersection gives 

 the per cent of moisture in 

 the air. 



Measurement of Rainfall. Rainfall is measured by a rain 

 gauge. This is a device for catching the rain over a certain area 



