New York AGricutrukaL EXpEkIMENT STATION. 199 
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 
