Note 765 — Chapter V · (continued 2)

As I am writing an account of Black's views, and not a criticism of them, I shall give them, without comment, in his own words, and in the words of one of his pupils. ‘Here we can also trace another magnificent train of changes, which are nicely accommodated to the wants of the inhabitants of this globe. In the equatorial regions, the oppressive heat of the sun is prevented from a destructive accumulation by copious evaporation. The waters, stored with their vaporific heat, are thus carried aloft into the atmosphere, till the rarest of the vapour reaches the very cold regions of the air, which immediately forms a small portion of it into a fleecy cloud. This also further tempers the scorching heat by its opacity, performing the acceptable office of a screen. From thence the clouds are carried to the inland countries, to form the sources in the mountains, which are to supply the numberless streams that water the fields. And, by the steady operation of causes, which are tolerably uniform, the greater part of the vapours pass on to the circumpolar regions, there to descend in rains and dews; and in this beneficent conversion into rain, by the cold of those regions, each particle of steam gives up the 700 or 800 degrees of heat which were latent in it. These are immediately diffused, and soften the rigour of those less comfortable climates.’ … ‘I am persuaded that the heat absorbed in spontaneous evaporation greatly contributes to enable animals to bear the heat of the tropical climates, where the thermometer frequently continues to show the temperature of the human body. Such heats, indeed, are barely supportable, and enervate the animal, making it lazy and indolent, indulging in the most relaxed postures, and avoiding every exertion of body or mind. The inhabitants are induced to drink large draughts of diluting liquors, which transude through their pores most copiously, carrying off with them a vast deal of this troublesome and exhausting heat. There is in the body itself a continual laboratory, or manufacture of heat, and, were the surrounding air of such a temperature as not to carry it off, it would soon accumulate so as to destroy life. The excessive perspiration, supplied by diluting draughts, performs the same office as the cold air without the tropics, in guarding us from this fatal accumulation.’ Black's Lectures, vol. i. pp. xlvi. 214.