9 2 



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 stand- 

 ard 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.* For convenience of illus- 

 tration 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 favor- 

 able 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 pre- 

 sented 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 es- 

 timated, 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 pro- 

 cess 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 require- 

 ments 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 evapora- 

 tion, as temperature, humidity, and the character of the prevail- 

 ing winds. 



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



* 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 re- 

 quired 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). 



