260. The proper degree of humidity of a fresh-air supply varies in different countries. In the United States, the dew point of the air when delivered into rooms should be about 40° F. This gives about 40 per cent. of humidity when the air has a temperature of 70° F.

Thus, if the air is fully saturated with moisture when at a temperature of 40°, its humidity will be correct when it reaches the temperature of 70°, no matter how much it may have been heated in the meantime. It is seldom necessary to raise the dew point any higher, except for certain manufacturing purposes, such as in weaving rooms, etc.

261. The methods commonly used for moistening air consist in passing it through a spray of water, or over the surface of water contained in evaporating pans, or by injecting steam into the air-current.

A very convenient method of humidifying a current of air is to inject steam into it. The jet should blow at right angles to the direction of the current, so that the steam will diffuse readily throughout the whole mass of air. The orifices usually required are very small, but they should never be made less than 1/32 inch in diameter, because such fine holes are very liable to become clogged and cease to operate.

The weight of steam that will be discharged into the atmosphere, per hour, through jets of small diameter, the pressure being not lower than about 20 pounds per square inch, absolute, is found to be as follows:

for each pound of steam pressure.

At lower pressures, the rate of discharge becomes slower and cannot be conveniently computed.