DEW POINT. In the article on Dew, we showed the manner in which dew is deposited, and some of the phenomena connected there with. It will only be necessary here to say that the temperature at which dew begins to form is called the Dew Point. This is a temperature at which the air begins to part with its vapors, and this again is a varying one. For instance, knowing the temperature of the air, if the bulb of a thermometer he placed on grass which quickly loses beat, the difference between the first degree of heat shown by the thermometer, and that shown when dew begins to form, is the Dew Point. Now, if at the time of placing the thermometer in the grass another be suspended from two to four feet above, the difference shown will sometimes be as much as 10° to 15°, if the bulb is protected from radiation by having some non radiating substance placed above it, as tin foil or paper. Again, in the deposition of dew, bright substances do not acquire moisture so quick as others; the reason is, they do not part with heat so readily as some others. Thus the heaviest dew will, of course, form most quickly on those substances that part with their heat easily. The simple means for testing the Dew Point we have shown is by no means accu rate. There are a number of hygrometers in use, some of them costly and intricate. The degree of cold necessary to be acquired by objects, before they can have dew deposited on them, can always be known as follows: Take a thin tumbler of glass, having polished sides; fill this about half full of ice•water. Plunge into it the
bulb of a thermometer, and the moment a film of dew or mist is seen to form on the polished outside surface, read the degree at which the thermometer stands, and this will be the dew point. The temperature at which atmospheric vapor condenses to form dew is generally several degreeS below the temperature of the atmos phere. But this is only the case during clear weather, since, when there is a fog, or a rain, the dew point will be found to correspond with the temperature of the air; showing that any cause which contributes to bring down the atmospheric temperature to the dew point, will i directly promote the condensation of its vapor or moisture into mist, cloud, rain, snow, or hail. The many relations which the dew point, or degree at which vapor condenses, holds with atmos pheric phenomena, may be understood from this; and it must be borne in mind that the dew point is almost continually rising or falling, like the temperature of the atmosphere, being usually, in clear weather, some four, six, eight, or ten degrees lower than common air, as indicated by the thermometer.