New York Agkiclltukal Expekimkjst Station. 11)9 



the temperature indicated by the dry and wet thermometers. 

 When the two thermometers indicate the same temperature, 

 then there is no evaporation taking place from the bulb of the 

 wet thermometer, because the air is saturated with moisture, 

 that is, holds as water-vapor all it can at that temperature. If 

 the moisture is increased beyond this point or if the tempera- 

 ture is lowered, some of the water-vapor will be condensed into 

 visible drops of water. 



In order to use a hygrometer for the purpose of ascertaining 

 the proportion of moisture in air, we note first the temperature 

 indicated by the dry thermometer. Then we dip in water the 

 bulb (wrapped in muslin) of the wet thermometer, whirl it 

 vigorously for one or two minutes, and then quickly read the 

 temperature. The whirling is for the purpose of quickly causing 

 evaporation. It is well to repeat the whirling two or three 

 times, noting the temperature of the wet thermometer after 

 each whirling. The different temperature readings should agree 

 if the whirling operation is equally thorough each time. Pains 

 must be taken to keep the muslin about the bulb moist during 

 the different whirlings. After getting the temperatures of the 

 two thermometers, we subtract the number indicating the 

 temperature of the wet thermometer from the number showing 

 the temperature of the dry thermometer. Then we turn to pre- 

 pared tables of figures and find the column of figures, at the top 

 of which is the difference obtained by the foregoing subtraction. 

 If the exact figure is not there we take the one nearest it. 

 We then follow down this column until the figure is found 

 opposite the number in the left hand column which is the same 

 as the temperature indicated by the dry thermometer. The 

 number thus found indicates the relative amount of moisture in 

 the air, or percentage of saturation; that is, how much moisture 

 the air actually holds compared with what it could hold at that 

 temperature if saturated. 



The preceding statements can be better understood by use of 

 a specific illustration. Suppose we find by actual trial that the 



