meteorology. Accordingly many experiments have been made to determine the point by different philosophers. No person has succeeded so completely as Mr. Dalton. But many curious particulars had been previously ascertained by the labours of Richman, Lambert, Wallerius, Leidenfrost, Watson, Saussure, Deluc, Kirwan, and others. 1. The evaporation is confined entirely to the surface of the water: hence it is in all cases proportional to the surface of the water exposed to the atmosphere. Much more vapour of course rises in maritime countries, or those interspersed with lakes, than in inland countries. 2. Much more vapour rises during hot weather than during cold weather. Hence, the quantity evaporated depends in some mea. sure upon temperature. The precise law has been happily dis. covered by Mr. Dalton. This philosopher took a cylindrical vessel of tin, whose diameter was 34 inches, and its depth 2 inches; filled it with water, and kept it just boiling for some time. The loss of weight in the minute was 30 grains, when the experiment was made in a close room without any draught of air; 35 grains when the vessel was placed over fire in the usual fire. place, there being a moderate draught of air, and the room close; 40 with a brisker fire, and a stronger draught; and when the draught was very strong, he supposes the evaporation might amount to 60 grains in the minute. At the temperature of 180°, the quantity evaporated was of what was lost at 212°. At 164 it was of that at 212°. 152... 138...... And in general the quantity evaporated from a given surface of water per minute at any temperature, is to the quantity evaporated from the same surface at 212°, as the force of vapour at the first temperature is to the force of vapour at 212°. Hence, in order to discover the quantity which will be lost by evaporation from water of a given temperature, we have only to ascertain the force of vapour at that temperature. And by such examination we shall see that the presence of atmospheric air obstructs the evaporation of water; but this resistence is overcome in proportion te the force of the vapour. 3. The quantity of vapour which rises from water, even when the temperature is the same, varies according to circumstances. It is least of all in calm weather, greater when a breeze blows, and greatest of all with a strong wind. The following Table, drawn up by Mr. Dalton, shows the quantity of vapour raised from a circular surface of six inches in diameter in atmospheric temperatures. The first column expresses the temperature; the second, the corresponding force of vapour; the other three columns give the number of grains of water that would be evapo. rated from a surface of six inches in diameter in the respective temperatures, on the supposition of there being previously no aqueous vapour in the atmosphere. These columns present the extremes and the mean of evaporation likely to be noticed, or nearly such; for the first is calculated upon the supposition of 35 grains loss per minute from the vessel of 34 inches in diameter; the second 45, and the third 55 grains per minute *. 4. Such is the quantity of vapour which would rise in different circumstances on the supposition that no vapour previously existed in the atmosphere. But this is a supposition which can never be admitted, as the atmosphere is in no case totally free from vapour. It has been shown in what manner the force of the vapour, existing in the atmosphere, may be detected by the use of Mr. Dalton's very simple apparatus. Now when we wish to ascertain the rate at which evaporation is going on, we have only to find the force of the vapour already in the atmosphere, and subtract it from the force of vapour at the given temperature. The remainder gives us the actual force of evaporation; from which, by the Table, we readily find the rate of evaporation. Thus, suppose we wish to know the rate of evaporation at the temperature 59°. From the Table we see that the force of vapour at 59° is 0·5, or th its force at 212°. Suppose we find by trials that the force of the vapour already existing in the atmosphere is 0-25, or the half of th. To ascertain the rate of evaporation, we must subtract the 0-25 from 0.5; the remainder 0.25 gives us the force of eva. poration required; which is precisely one-half of what it would be if no vapour had previously existed in the atmosphere. By the Table we see, that on that supposition a surface of six inches diameter would loose one grain by evaporation per minute, instead of two grains, which would have been converted into vapour if no vapour had previously existed in the atmosphere. If the force of the vapour in the atmosphere had amounted to 0.5, which is equal to the force of vapour at the temperature of 59°, in that case no vapour whatever would rise from the water: and if the force of the vapour already in the atmosphere exceeded 0·5, instead of evaporation, moisture would be deposited on the surface of the water. These general observations, for all of which we are indebted to Mr. Dalton, account in a satisfactory manner for most of the anomalies which had puzzled preceding philosophers; and include under them the less general laws which they had discovered. We must consider the discoveries of Mr. Dalton as the most important additions made to the science of meteorology for these many years. 5. As the force of the vapour actually in the atmosphere is seldom equal to the force of vapour of the temperature of the atmosphere, evaporation, with a few exceptions, may be considered as constantly going on. Various attempts have been made to ascertain the quantity evaporated in the course of a year; but the difficulty of the problem is so great, that we can expect only an approximation towards a solution. From the experiments of Dr. Dobson of Liverpool, in the years 1772, 1773, 1774, and 1775, it appears, that the mean annual evaporation from the surface of water amounted to 36.78 inches *. The proportion for every month was the following: Mr. Dalton found the evaporation from the surface of water, in one of the driest and hottest days of summer, rather more than 0.2 of an inch. If we believe Mr. Williams, the evaporation from the surface of land, covered with trees and other vegetables, is one-third greater than from the surface of water; but this has not been confirmed by other philosophers. From his experiments + it appears, that in Bradford, in New England, the evaporation during 1772 amounted to 42.65 inches. But from the way that his experi. ments were conducted, the amount was probably too great. From an experiment of Dr. Watson, made on the 2d of June 1779, after a month's drought, it appears that the evaporation, from a square inch of a grass plot, amounted to 12 grains in an hour, or 28.8 grains in 24 hours, which is 0.061 of an inch. In another experiment after there had been no rain for a week, the heat of the earth being 110°, the evaporation was found almost twice as great, or=0·108 of an inch in the day ‡. The mean of these two experiments is 0.084 inches, amounting for the whole * Phil. Trans. vol. Ixvii.-Dalton, Manchester Memoirs, v. 358. Watson's Chemical Essays, iii. 54. |
