Page:Popular Science Monthly Volume 41.djvu/102

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92
THE POPULAR SCIENCE MONTHLY.

at Rothamsted was at the rate of 1,680,000 pounds of water per acre, or the equivalent of 7·4 inches of rainfall; and on the same "basis the exhalation from a crop of Indian corn, of 60 bushels per acre, would be equivalent to about 8·5 inches of rainfall.

So far as the expenditure of energy is concerned, it matters not whether water is changed to vapor in the process of exhalation by the crop or in evaporation from the soil, and the same standard of measurement will, therefore, be applicable in both cases.

Energy is measured in heat-units, and work is expressed in foot-pounds or in kilogramme-metres.[1] For convenience of illustration we will make use of another standard adoj)ted by engineers, which, although not as definite, is sufficiently accurate for our purpose.

From experimental data it has been found that, under favorable conditions, one pound of coal will evaporate from 6·60 to 8·66 pounds of water from an initial temperature of 32 Fahr., according to the quality of the coal used. If we assume that one pound of coal will evaporate 8·5 pounds of water under the conditions presented in crop-growing, our standard will be considerably above what is realized in ordinary steam-engines.

The energy required to vaporize the water exhaled by one acre of corn in its processes of growth, with a yield as above estimated, would, therefore, be represented by the heat produced in burning 226,500 pounds of coal, or over 113 tons. This may be expressed in another form, which will, perhaps, be more readily understood. We are told that "a good condensing engine of large size, supplied with good boilers, consumes two pounds of coal per horse-power per hour." The work involved in the process of exhalation from one acre of corn would, therefore, be equivalent to the work of more than twenty-five horses day and night, without cessation, for six months.

The same standards of measurement applied to the energy expended in evaporating water from the soil will give quite as striking results. With a sufficient rainfall to supply the requirements of a crop, the amount of water evaporated from the soil will vary, within certain comparatively narrow limits, with the amount and distribution of the rainfall, the capacity of the soil for heat, and the atmospheric conditions that influence evaporation, as temperature, humidity, and the character of the prevailing winds.

From the best evidence I can obtain, which need not here be


  1. The English heat-unit is the amount of heat required to raise one pound of water 1° Fahr. in temperature, and the French heat-unit, or calorie, is the amount of heat required to raise one kilogramme of water 1° C. in temperature.

    A foot-pound = one pound raised one foot.

    A kilogramme-metre = one kilogramme (2·2 pounds) raised one metre (3·28 feet).