few more warm days, may make his own estimates. In Roxbury,
Mr. I. P. Rand heats up a portion of his land, for the purpose of
raising early plants for the market, by means of hot water carried by
iron pipes under the surface of the ground. In this manner he heats an
area equal to 100 feet by 12 feet, by burning about one ton of coal a
month. The increase of temperature which, in this case, is caused by
that amount of coal, can, in the absence of direct measurement, only be
estimated; but it, probably, will average about 30 deg., day and night,
throughout the month. In an acre the area is 36.4 times as great as that
heated by one ton of coal; the cost being in direct proportion to the
area, 36.4 tons of coal would be required to heat an acre; which, at $6
per ton, would cost $217.40. To heat an acre through 10 deg., would cost,
then, $72.47. It may be of interest to consider how much coal would be
required to evaporate from an undrained field that amount of water which
might be carried off by under-drains, but which, without them, is
evaporated from the surface. It may be taken as an approximate estimate,
that the evaporation from the surface of an undrained retentive field,
is equal to two inches vertical depth of water for each of the months of
May, June, July, and August; which is equal to fifty-four thousand three
hundred and five gallons, or eight hundred and sixty-two hogsheads per
acre for each month. If this quantity of water were evaporated by means
of a coal fire, about 22-2/3 tons of coal would be consumed, which, at
$6 a ton, would cost $136. The cost of evaporating the amount of water
which would pass off in one day from an acre would be about $4.53. It is
probable that about half as much water would be evaporated from
thorough-drained land, though, by some experiments, the proportion has
been made greater--in which case the loss of heat resulting from an
excess of moisture evaporated from undrained retentive land, over that
which would be evaporated from drained land, would be equal to that
gained by 11-1/3 tons of coal, which would cost $68; and this for each
acre, in each of the three months. At whatever temperature a liquid
vaporizes, it absorbs the same total quantity of heat.

The latent heat of watery vapor at 212 deg. is 972 deg.; that is, when water at
212 deg. is converted into vapor at the same temperature, the amount of heat
expended in the process is 972 deg.. This heat becomes latent, or insensi