AGRICULTURAL METEOROLOGY. — 147 
differences of degree in temperature. When exposed for hours to the 
sun they never attain the same degree of heat—the black earths acquir- 
ing the greatest heat aud the lighter ones remaining cooler. 
2. If we expose earths of the same kind in a dry and a wet state to 
the sun, the wet earth never attains an equal degree of heat with the 
dry. The depression of temperature arising from evaporation amounts 
to 114° to 1359 F. As long as the earths remain saturated with water 
they show little difference in their heat-acquiring powers, as they give 
off to the air, in this condition, nearly equal quantities of vapor in the 
same length of time; as they gradually dry, the difference in temperature 
increases. Light-colored earths, with great water-containing powers, 
acquire heat slowly, while dark-colored sand and slates, containing less 
moisture, become heated more quickly and powerfully. 
3. The different ingredients which enter into the composition of soils 
have, in themselves, far less influence on the capacity of soils to become 
warmed by the sun than their color and dryness. If we impart, artifi- 
cially, to earths the same color and expose them in a similarly dry con- 
dition to the sun, the differences in temperature will be inconsiderable; 
so that the various capacities of earths, in their natural state, for re- 
ceiving heat from the sun, depend particularly upon their color and dry- 
ness. 
4, The inclination of the ground toward the sun has a very consider- 
able influence on the degree of heat which the soil receives from its 
tays, and the greater warmth is produced as the incidence of the ray 
approaches more nearly to aright angle, or 90°, with the surface. If 
the actual increase of the temperature in the sun over that in the shade 
be between 45° and 65°, as is often the case on clear summer days, this 
increase would: be only half as great if the same light spread itself, in a 
more slanting direction, over a surface twice as extended. This is the 
reason why beat so frequently increases on the slopes of mountains and 
rocks which have an inclination toward the south. When the sun is at 
an elevation of 60° above the horizon, as is more or less the case toward 
noon in the middle of summer, the sun’s rays fall on the slopes of 
mountains which are raised to an inclination of 30° to the horizon, at a 
right angle; but even in the later months of summer the sun’s rays 
frequently fallon them under aright angle in cases where the slopes 
are yet steeper. Such declivities, particularly in the geographical lati- 
tude of Germany, are therefore peculiarly suited to the cultivation of 
plants which require a high temperature, such as the vine. By an 
accurate comparison of the power of the sun’s rays to warm the soil, 
with reference to the different seasons, we shall perceive more distinctly 
the influence of the different inclinations of the ground toward the sun. 
Capacity of soils to develop heat within themselves on being moistened.— 
Powdery substances in general, and consequently the earths, possess the 
property of developing warmth when moistened while in a dry state; 
but in nature they are scarcely ever found in this perfectly dry condi- 
tion. The rain falling in warm seasons is many degrees colder than the 
lower stratum of the atmosphere and the upper surface of the earth, 
which it immediately moistens; so that the earth in hot weather becomes 
rather cooler than otherwise by the rain. When the earth has previously 
been very dry, the cooling of it by the rain ean only be reduced about 
one degree of Fahrenheit, and this would have little effect on vegetation, 
and in the colder seasons, when the earth is already damp, so slight a 
development must be inappreciable. } 
Galvanic and electrical relations of the earths —The pure earths, as 
sand, lime, magnesia, and gypsum, in their dry state, are non-conductors; 
