122 
On the Lifluence of Water 
given weig^ht of steam in water, when it is found that a pound of 
steam will raise about 6 lbs. of water from 50° to the boiling- 
point. 
Water is vaporizable at all temperatures when exposed to the 
atmosphere. Its expulsion from the earth does even, under 
certain circumstances, continue when the atmosphere is replete 
with moisture, or at what is termed the dew-point. And it is 
most important to observe that at however low a temperature the 
water in the soil, or that of the atmosphere incumbent on it may 
be, at which vapour is formed and expelled, the same amount 
of heat is carried off by a given weight of vapour, as if it had been 
generated in the open vessel over the fire above referred to, or in 
the close boiler of a high-pressure steam-engine. A practical 
confirmation of the truth of this law has been obtained by eva- 
porating water under widely different pressures, when it appeared 
that the same weight of fuel (or measure of heat) was consumed 
in converting equal bulks of water into steam at all those different 
pressures. It is ascertained that it requires as much heat as 
2 or 3 oz. of coal will produce, to convert I lb. of water into 
vapour : it is, therefore, evident what an enormous quantity of 
heat must be taken from the soil in cases where water is allowed 
to remain stagnant upon it till it evaporates. 
As heat is generally considered to be an imponderable body, 
we are without the means of ascertaining directly, by weight or 
measure, the quantity of heat absorbed from soil by the evapora- 
tion of water. The following illustration of it will, however, be 
familiar enough to the mind of the engineer, and will also, I 
think, enable intelligent farmers to form an idea of its immense 
amount. 
If we suppose the rain falling on the surface of an acre of land 
in a year to be 30 inches in perpendicular depth, it would amoimt 
to 108,900 cubic feet = 3038 tons; which, spread over a twelve- 
month, gives an average of 298 cubic feet =: 8^ tons, or 18,647 lbs. 
per diem. This weight of water would require, for its diurnal 
evaporation — supposing it were all carried off by that means — 
the combustion of about 24 cwt. of coals, as ordinarily used under 
a steam-boiler, or 1 cwt. per hour per acre throughout the 
year ! We thus obtain some idea of the abstraction of heat from 
land under the circumstances of perfect aqueous repletion and 
stagnation, and there are too many soils approaching to them. We 
may also imagine the depression of the terrestrial temperature 
consequent on the abstraction of so much heat from the mass of 
the soil — a de})ression which must ever be in projiortion to the 
excess of water present in a soil, over and above the due comple- 
ment required for the supply of vegetation. Soils in that state 
must necessarily be very cold in the spring months, and much 
