For it will be perceived that an ordinary
subtraction of the degrees of temperature on a wet thermometer, which
had cooled down by evaporation, from the actual temperature indicated
by a dry thermometer, will not give us the dew-point.

For example,--if a free or dry thermometer indicates 63 deg., and the one
with the wet bulb has by evaporation cooled down to 54 deg., the
difference would be 9 deg.. The dew-point would not be 54 deg., but that
degree to which the mercury would fall in the free thermometer, for
the atmosphere to become saturated with the quantity of moisture then
actually existing in it. It would be 46.8 deg..

This dew-point, which figures so largely in all well-kept
meteorological reports, is the key to many important conditions of the
atmosphere, affecting health, vegetation, and climate.

It is found that the air at different degrees of heat has different
degrees of elasticity, different degrees of tension, and different
degrees of capacity to hold vapor. Dalton, by a series of experiments
with barometer-tubes, into which he introduced air and vapor at
certain temperatures, found what its force was upon the mercurial
column from degree to degree. He also experimentally determined the
ratio of the weight of moisture and of air, the former being five-eights
of the latter,--in other words, how many grains of moisture
additional could be held by the air, advancing from degree to degree
of temperature. This being ascertained, a table of factors was
constructed, in other words, a set of figures contrived, which should,
by a multiplication of the subtracted difference between the range of
the dry bulb and the wet bulb of the thermometers, furnish the amount
of deduction from the former which would indicate the dew-point, or
the point to which the mercury in the dry thermometer must fall to
show how much more moisture the air could hold without its
condensation. These tables of factors have been constructed at the
Greenwich Observatory, and are generally used.

The Hygrometer, invented by Mr. Daniell, gives the dew-point by
inspection.

It is an error to suppose that dew falls like rain from the air; it
forms on the body which is cooled down below the temperature of the
air. It differs in quantity with the radiating or cooling surface;
that which has absorbed and retained the most heat during the day
radiates the most at night and furnishes the most cold in return.

Hoar-frost, such as we find on our