122 Princtples of Plant Culture. 
the air corresponds to the compression of the sponge. Now, 
the atmosphere always contains more or less water in the 
form of watery vapor, and the temperature at which any 
portion of the atmosphere, on cooling, begins to precipi- 
tate a part of its water, is called the dew point. The tem- 
perature of the dew point will of course depend upon the 
amount of water the air contains. When the dew point is 
above the freezing point of water (32° F., O°C.), tbe preci- 
pitation is in the form of dew, or rain; but when it is below 
the freezing point of water, the precipitation is in the form 
of hoarfrost or snow. One more principle needs to be 
explained, and we are ready to understand 
207. How Frost may be Foretold. We know that 
sprinkling the floor of a room cools the air, even though the 
water used is no cooler than the air of the room. This is 
because the air, in absorbing watery vapor, absorbs heat, 
but this heat is set free again when the watery vapor is 
precipitated. A steam radiator gives out heat because the 
steam within it is condensing into water. It follows that 
when the dew point of the atmosphere is reached, a very 
considerable amount of latent heat is given off, which 
checks the fall of temperature. The temperature of still air, 
therefore, rarely falls much below the dew point, and since the 
latter depends at any given temperature upon the amount 
of moisture in the air, if we have an instrument capable of 
indicating both the temperature and the moisture of the air, 
we may compute the lowest temperature to which the atmos- 
phere will be likely to descend during any given night. 
208. The Sling Psychrometer (psy-chrom’-e-ter) en- 
ables us to do this. This instrument consists of two thermom- 
eters, known to be accurately graduated, attached to a board 
