whole
body of soil till moisture is apparent at the surface. Put in your soil at
60 deg., a reasonable summer heat for nine inches in depth, your water at 47 deg.,
the seven inches' temperature of Mr. Parke's undrained bog; the attracted
water will ascend at 47 deg., and will diligently occupy itself in attempting
to reduce the 60 deg. soil to its own temperature. Moreover, no sooner will
the soil hold water of attraction, than evaporation will begin to carry it
off, and will produce the cold consequent thereon. This evaporated water
will be replaced by water of attraction at 47 deg., and this double cooling
process will go on till all the water in the water-table is exhausted.
Supply water to the saucer as fast as it disappears, and then the process
will be perpetual. The system of saucer-watering is reprobated by every
intelligent gardener; it is found by experience to chill vegetation;
besides which, scarcely any cultivated plant can dip its roots into
stagnant water with impunity. Exactly the process which we have described
in the flower-pot is constantly in operation on an undrained retentive
soil; the water-table may not be within nine inches of the surface, but in
very many instances it is within a foot or eighteen inches, at which level
the cold surplus oozes into some ditch or other superficial outlet. At
eighteen inches, attraction will, on the average of soils, act with
considerable power. Here, then, you have two obnoxious principles at work,
both producing cold, and the one administering to the other. The obvious
remedy is, to destroy their _united_ action; to break through their line
of communication. Remove your water of attraction to such a depth that
evaporation cannot act upon it, or but feebly. What is that depth? In
ascertaining this point we are not altogether without data. No doubt depth
diminishes the power of evaporation rapidly. Still, as water taken from a
30-inch drain is almost invariably two or three degrees colder than water
taken from four feet, and as this latter is generally one or two degrees
colder than water from a contiguous well several feet below, we can hardly
avoid drawing the conclusion that the cold of evaporation has considerable
influence at 30 inches, a much-diminished influence at four feet, and
little or none below that depth. If the water-table is removed to the
depth of four feet, when we have allowed 18 inches of attraction, we shall
still have 30 inches of defence against