This increased fertility of the soil renders it more healthy by diminishing its moisture and raising its temperature. One cubic foot of water in the process of evaporation deprives three millions of cubic feet of air of one degree of temperature. An undrained field growing rushes has a permanent temperature from four to six degrees lower than an adjoining field drained and growing wheat. By draining and manuring, by throwing down fences, by removing trees, by clearing underwood, and by promoting the free aëration of the soil, the temperature of large tracts of land in the north of England has been permanently raised three degrees. Thus that very culture of the earth, by which it is made to yield the largest amount of food, increases its salubrity as an abode for man, and lessens at their source the main causes of epidemics. This increased production has been obtained by a proportionally small addition to labour; for while the quantity of land brought under cultivation, and its produce, have been increasing at a rate of which there is no similar example in any age or country, the relative number of persons employed in agriculture has been as steadily decreasing. As long as the labour of a man applied to the cultivation of the soil is capable of producing only a bare subsistence for himself, there can be no advance in civilization. But when two men can produce subsistence for three, the labour of the third can be set free for the production of surplus articles, which add to the sum of the general convenience, and from that moment the community takes a start in the career of improvement. From a comparison of occupations taken in 1831, it appears that, at that time, the division of labour among the people was such that one person raised nearly all the food of home production consumed by four persons.[18] Footnote 18: Porter’s Progress of the Nation, Chap. III. Were the remaining three idle? Mediately or immediately they were engaged in producing clothing, or fuel, or machinery, economizing the production of both; and busily and well they worked.