ae 
thermometer and a dry-bulb thermometer mounted side by side. If it 
is reasonably protected from radiation and placed in an air stream of 
at least 7 miles per hour, the two thermometor readings and the 
barometric pressure are all the information necessary for conputing 
the humidity. Table 3 gives the formula by which the vapor pressure, 
€, in the air may be calculated. 
Vapor pressure may alternatively be comouted from relative hu- 
midity and temperature, or vice versa. Relative humidity, f, is 
defined as the ratio of the actual vapor pressure to the saturation 
vapor pressure, em, 
Radiation laws 
The rate at which a body radiates heat, including light, depends 
on its temperature. Bodies of the same size and at the same temper- 
ature but of different substance emit radiation at different rates, 
radiator. An ideal radiator furthermore absorbs all radiation falling 
upon it. The total radiation, EH, from an ideal radiator is propor- 
tional to the fourth power of its absolute temperature, T, according 
to Stefan's law, 
eet 
The constant s has the value 1,079 2) 20r/ cal emm2 daye! (deg Ayes 
€ecalorics per square centimeter per day per degree absolute to the 
fourth power}. Many natural bodies approximate closely an ideal 
Fadiator. For instanee the sun emits practically the same radiation 
as would an ideal radiator at 5600°A, 
The spectral distributich also of the radiation from a ideal 
radiator depends only on its temperature. The distribution is a 
