6 
meteorology' of 1897. 
temperature of evaporation. The drier the air, the lower 
does this thermometer read. In some of the summer 
months the difference is found to be as great as 15° or 20°. 
Attention has been called to the fact that this temperature 
would indicate approximately the temperature which the 
human body experiences, as the body is approximately in 
the condition of an instrument that is moist, and whose 
temperature is less than that of the air. It has therefore 
been proposed to call this the Sensible temperature. This 
term, which is free from objection during the summer 
months, is entirely misleading for temperatures below freez- 
ing. The observations are made by a Sling Pyschrometer 
consisting of two exactly similar thermometers placed side 
by side. These are swung in the air, in order to cause the 
rapid evaporation which will only take place by the renewal 
of air in contact with the bulbs. 
§26. The dew point and the relative humidity are ob- 
tained by calculation from the observations on the dry 
and wet bulbs. This is done by the aid of tables, still in 
manuscript, prepared in 1889, and based on Terrel’s re- 
searches. By the temperature of dew point, is meant the 
temperature at which the air is saturated with the amount 
of moisture it contains. At all temperatures the air con- 
tains moisture. Ordinarily this is invisible, and the air does 
not contain all that it will hold, or it is not saturated. At 
low temperatures it will not hold nearly as much as at high 
temperatures. In consequence, when the air is cooled 
without loss of moisture, it approaches saturation, or the 
relative humidity is increased, while the dew point remains 
the same. When saturation is reached, (when the relative 
humidity is 100 per cent, or when the dew point is the same 
as the air temperature) any additional cooling will cause 
some of the moisture to be condensed into a visible form. 
When a large mass of air is cooled, it may be as a cloud ; or 
if the air is cooled by contact with some body below the 
dew point, it may be as dew or frost. The temperature of 
dew point thus indicates the temperature of the air at which 
the amount of moisture actually present causes saturation. 
Thus every time when the dew point is the same, the abso- 
lute amount of moisture present is the same. The relative 
humidity expresses the percentage that the moisture actu- 
ally present bears to the amount of moisture which would 
saturate the air at the temperature shown by the dry bulb. 
A relative humidity of 100 per cent indicates complete satu- 
ration. 
