" " " highest, August 17 and 18, 64 deg.
" " " at four ft. depth, 17 days in Aug. 60 deg.
" " " of the air, at 3, P. M., highest 90 deg.

The temperature of falling rain, however, in the hot season, is many
degrees cooler than the lower stratum of the atmosphere, and the surface
of the earth upon which it falls. The effects of rain on drained soil,
in the heat of Summer, are, then, two-fold; to cool the burning surface,
which is, as we have seen, much warmer than the rain, and, at the same
time, to warm the subsoil which is cooler than the rain itself, as it
falls, and very much cooler than the rain-water, as it is warmed by its
passage through the hot surface soil. These are beautiful provisions of
Nature, by which the excesses of heat and cold are mitigated, and the
temperature of the soil rendered more uniform, upon land adapted, by
drainage, to her genial influences.

Upon the saturated and water-logged bog, as we have seen, the effect of
the greatest heat is insufficient to raise the temperature of the
subsoil a single degree, while the surface may be burned up and
"shrivelled like a parched scroll."

Drainage also raises the temperature of the soil by the admission of
warm air. This proposition is closely connected with that just
discussed. When the air is warmer than the soil, as it always is in the
Spring-time, the water from the melting snow, or from rain, upon drained
land, passes downward, and runs off by its gravitation. As "Nature
abhors a vacuum," the little spaces in the soil, from which the water
passes, must be filled with air, and this air can only be supplied from
the surface, and, being warmer than the ground, tends to raise its
temperature. No such effect can be produced in land not drained, because
no water runs out of it, and there are, consequently, no such spaces
opened for the warm air to enter.

Drainage equalizes the temperature of the soil in Summer by increasing
the deposit of dew. Of this we shall speak further, in a future
chapter.

_Drainage raises the temperature in Spring by diminishing evaporation._
Evaporation may be defined to be the conversion of liquid and solid
bodies into elastic fluids, by the influence of caloric.

By heating water over a fire, bubbles rise from the bottom of the
vessel, adhere awhile to the sides of it, and then ascend to the
surface, and burst and go off in visible vapor, or, in other