h, at first sight, appear to be entirely
chemical, are also in reality partly mechanical. It will be necessary
for us, therefore, to consider shortly the mechanical methods of
improving the soil.

_Draining._--By far the most important method of mechanically improving
the soil is by draining--a practice the beneficial action of which is
dependent on a great variety of circumstances. It is unnecessary to
insist on the advantage derived from the rapid removal of moisture,
which enables the soil to be worked at times when this used to be almost
impossible, and other direct practical benefits. Of its more strictly
chemical effects, the most important is probably that which it produces
on the temperature of the soil. It has been already remarked that the
germination of a seed is dependent on the soil in which it is sown
acquiring a certain temperature, and the rapidity of the after-growth of
the plant is, in part at least, dependent on the same circumstance. The
necessary temperature is speedily attained by the heating action of the
sun's rays, when the soil is dry; but when it is wet, the heat is
expended in evaporating the moisture with which it is saturated; and it
is only after this has been effected that it acquires a sufficiently
high temperature to produce the rapid growth of the seeds committed to
it.

The extent to which this effect occurs may be best illustrated by
reference to some experiments made by Schuebler, in which he determined
the temperature attained by different soils, in the wet and dry state,
when exposed to the sun's rays, from 11 till 3 o'clock, in the latter
part of August, when the temperature in the shade varied from 73 deg. to
77 deg..

+-----------------------+-------+-------+
| Description of Soil. | Wet. | Dry. |
+-----------------------+-------+-------+
| | Degs. | Degs. |
| Siliceous sand | 99.1 | 112.6 |
| Calcareous sand | 99.3 | 112.1 |
| Sandy clay | 98.2 | 111.4 |
| Loamy clay | 99.1 | 112.1 |
| Stiff clay | 99.3 | 112.3 |
| Fine bluish-grey clay | 99.5 | 113.0 |
| Garden mould | 99.5 | 113.5 |
| Arable soil | 97.7 | 111.7 |
| Slaty marl | 101.8 | 115.3 |
+-----------------------+-------+-------+

In a soil which is naturally dry or has been drained, the superfluous
moisture escapes by the drains, and only that comparatively small
quantity which is retained by capillary