Dr Halley attempted to ascertain the quantity which evaporates from the surface of water, in natural tem peratures, by exposing a circular surface of about eight inches diameter, to a heat equal to that of a summer's day, and found it to be at the rate of six ounces in 24 hours, or of an inch deep in 12 hours. By this ex periment, each square foot of surface yields in vapour daily about half a wine pint ; a square mile 6900 tons ; a square degree 33 millions of tons: Phil. Trans. No. 189.) A surface of eight square inches, evaporated by the natural temperature of the air, without exposure to wind or sun, in the course of a whole year, 16292 grains of water, or about 64 cubic inches ; consequently the depth of water evaporated in that time was eight inches. Such experiments, however, are of little value ; and no general conclusion can be drawn from them respecting the average quantity evaporated from the surface of the earth. The experiments of Mr Hoyle and Mr Dalton were conducted with greater precision; but still they must be regarded as only of local application. A cylin drical vessel of tinned iron, ten inches in diameter, and three feet deep, having tubes soldered to it for convey ing off into bottles the water which it received, was bu ried in the ground in an open situation, and then filled with gravel, sand, and soil. The whole being covered with grass, and other vegetables, it was allowed to re ceive the rain, and to suffer evaporation from the sur face, as in ordinary circumstances. A register was kept of the water which made its way through the soil into the bottles ; and a raingage of equal surface was placed close by, for the sake of comparison. The following Table exhibits the results obtained, and also the mean evaporation from a surface of water, for three succeed, ing years.
The writer of this article, employed the following method of discovering, by evaporation, the quantity of latent caloric existing in steam or vapour. A quantity of pure water, equal in weight to 550 grains, was placed in a small glass cup, weighing 440.68 grains, along with a pocket thermometer, the weight of which was 59.32 grains. The whole was disposed as in the experiment described under the article COLD; and the exhaustion was then carried on slowly and cautiously, to prevent any part of the water from sparkling over, by too violent an ebullition. In four minutes the water was reduced in temperature from 47° to 30r ; and on the air being ad mitted, was found to have lost 12.75 by evaporation. It was ascertained, by a calculation founded upon the mean result of several experiments, that the thermometer and the cup which contained the water, would, in point of temperature, have produced the same effect as 130 grains of water ; so that the whole quantity of water cool ed down might be considered as equal to '2.75 12.75, or 667.25 grains. This gives 1 r as 667.2a 524 the ratio of the portion of water evaporated, to that cool ed down from 47° to 303° or 16r ; and as this quantity must have combined with a dose of caloric, the abstrac tion of which was sufficient to reduce the temperature of 667.25 grains of water 16e, the vapour must have com
bined with 521X163, or A slight correction must be applied to the result, to make allowance for the in gress of caloric during the time of the experiment, which was observed to be at the rate of 1° in 6 minutes, or 3of a degree in 4 minutes ; so that the water might be consi dered as having been actually cooled down 17e. This correction being made, the quantity of latent caloric which combined with the vapour is 898° nearly. The mean result of three experiments, conducted in the same manner, was Having stated the results of these experiments, we may mention, that the process of forming ice in the East Indies, when the temperature of the air is above the freezing point, by exposing water in shallow unglazed earthen vessels, has been usually explained by ascribing - the reduction of temperature to the evaporation of the water ; and that this must, in many cases, contribute to the effect, cannot be doubted, since the evaporation of part of the water exposed, if the process were in stantaneous, would be capable of reducing its tempera ture 10°. But Dr Wells, in his Esaay on Dew, has en deavoured to slim, that the principal share of the effect is owing to radiation. By exposing water, in circum stances similar to those in which ice is formed in Ben gal, this gentleman observed, that the water, instead of suffering a loss of weight by evaporation, became, in some cases, actually heavier by the deposition of mois ture from the atmosphere; so that, in such instances, the effect of evaporation in cooling the water, must have been more than counteracted by the caloric imparted by the condensed vapour. One experiment on this subject which he relates, was performed on the evening of the 16th of October. With the view of imitating the me thod of making ice in the East Indies, he caused a pit, 43 feet long, 3 feet wide, and 2 feet deep, to be dug in the middle of a garden, and clean dry straw to be after wards strewed to the height of a foot over the bottom of it. On the straw were next placed a number of small shallow earthen pans, a part of which were glazed, and a part unglazed. In the last place, all the pans were filled with soft water, which had been boiled on the same evening. On the night alluded to, ice appeared in the pans, when the temperature of the air, at the height of SA feet, was, according to a naked thermometer, 37°. A dry earthen pan was placed among those which con tained water, and the inside of its bottom was found to be as much colder than the air, as the water was in the other pans, before ice appeared in them. Moisture was attracted by this pan during the night, which was after wards converted into a film of ice.