ontact with it, small pieces of black mirror, or any
substance having a surface that shows dewing easily. In this way a small
area of the surface of the earth is converted into a hygroscope, and
these test surfaces tell us whether the ground is cooled to the dew-point
or not. So long as they remain clear and undewed, the surface of the soil
is hotter than the dew-point, and vapor is being given off, while if they
get dewed, the soil will also be condensing vapor. On all nights
observed, these test-surfaces kept clear, and showed the soil to be
always giving off vapor.

All these different methods of testing point to the conclusion that
during dewy nights, in this climate, vapor is constantly being given off
from grass land, and almost always from bare soil; that the tide of vapor
almost always sets outward from the earth and but rarely ebbs, save after
being condensed to cloud and rain, or on those rarer occasions on which,
after the earth has got greatly cooled, a warm moist air blows over it.
The results of the experiments are given, showing, from weighings, the
amount of vapor lost by the soil at night, and also the heat lost by the
surface soil.

It seems probable that when the radiation is strong, that soil,
especially if it is loose and not in good heat-communication with the
ground, will get cooled below the dew-point, and have vapor condensed
upon it. On some occasions the soil certainly got wetter on the surface,
but the question still remains, Whence the vapor? Came it from the air,
or from the soil underneath? The latter seems the more probable source;
the vapor rising from the hot soil underneath will be trapped by the cold
surface-soil, in the same way as it is trapped by grass over grass-land.
During frost, opportunities are afforded of studying this point in a
satisfactory manner, as the trapped vapor keeps its place where it is
condensed. On these occasions the under sides of the clods, at the
surface of the soil, are found to be thickly covered with hoar-frost,
while there is little on their upper or exposed surfaces, showing that
the vapor condensed on the surface-soil has come from below.

The next division of the subject is on dew on roads. It is generally said
that dew forms copiously on grass, while none is deposited on roads,
because grass is a good radiator and cools quicker, and cools more, than
the surface of a road. It is shown that the above statement is wrong, and
that dew really does form