The most exact set of experiments on the evaporation from the earth was made by Mr. Dalton and Mr. Hoyle, during 1796, and the two succeeding years. The method which they adopted was this : having got a cylindrical vessel of tinned iron, ten inches in diameter, and three feet deep, there were inserted into it two pipes turned downwards, for the water to run off into bottles : the one pipe was near the bottom of the vessel, the other was an inch from the top. The vessel was filled up for a few inches with gravel and sand, and all the rest with good fresh soil. It was then put into a hole in the ground, and the space around filled up with earth, except on one side, for the convenience of putting bottles to the two pipes ; then some water was poured on to sodden the earth, and as much of it as would was suffered to run through without notice, by which the earth might be considered as saturated with water. Fur some weeks tbe soil was kept above the level of the upper pipe, but latterly it was constantly a little below it, which precluded any water of through it. For the first year the soil at top was bare; but for the two last years it was covered with grass, the same as any green field. Things being thus circum stanced, a regular register ryas kept of the quantity of rain water that ran off from the surface of the earth through the up per pipe, (whilst that took place,) and also of the quantity of that which sunk down through the three feet of earth, and ran out through the lower pipe. A
rain guage of the same diameter was kept close by, to find the quantity of rain for any corresponding time. The weight of the water which ran through the pipes being subtracted from the water in the nit) guage, the remainder was consider ed as the weight of the water evaporat ed from the earth in the vessel. From these experiments it appears, that the quantity of vapour raised annually at Manchester is about 25 inches. If to this we add five inches for the dew, with Mr. Dalton, it will make the annual evapora tion 30 inches. Now, if we consider the situation of England, and the greater quantity of vapour raised from water, it will not surely be considered as too great an allowance, if we estimate the mean annual evaporation over the whole sur face of the globe at 35 inches. Now, 35 inches from every square inch, on the su perficies of the globe, make 94,450 cubic miles, equal to the water annually evapo rated over the whole globe. Was this prodigious mass of water all to subsist in the atmosphere at once, it would increase its mass by about a twelfth, and raise the barometer nearly three inches : but this never happens ; no day passes without rain in some part of the earth ; so that part of the evaporated water is constantly pre cipitated again. Indeed, it would be im possible for the whole of the evaporated water to subsist in the atmosphere at once, at least in the state of vapour. See Manchester Memoirs.