The principle of evaporation is the primary cause of all rain, mist, dew, &c. The ocean loses many millions of gallons of water hourly by evaporation. The Mediterranean alone is said to lose more by it, than it receives from the Nile, the Tiber, the Rhone, the Po, and all the other rivers that fall into it. When Dr. Halley made his celestial observations upon the tops of the mountains at St. Helena, he found that the quantity of vapor which fell there (even when the sky was clear) was so great, that his observations were thereby much impeded: his glasses were so covered with water through the condensation of the vapors, that he was obliged to wipe them every ten minutes. In order to determine, with some degree of accuracy, how much water would be raised in vapor in any space of time, he took a vessel of water salted to the same degree with that of sea-water, in which he placed a thermometer, and by means of a pan of coals brought the water to the same degree of heat as would be produced by the sun in summer: he then affixed the vessel of water with the thermometer in it, to one end of a pair of scales, and exactly counterpoised it with weights on the other. Then, at the end of two hours, he found by the alteration in the weight of the vessel, that a sixtieth part of an inch in the depth of the water was gone off in vapor; and therefore, in twelve hours, one-tenth of an inch would have gone off. From this experiment the Doctor calculates (in as accurate a manner as the subject will admit of) the quantity of water raised by evaporation from the Mediterranean Sea, to be at least five thousand two hundred and eighty millions of tons of water in a day; and from the river Thames twenty millions three hundred thousand tons per day, on the average. This water is conveyed by the winds to every part of the continents: these it fertilizes in the form of rain, and afterwards supplies the rivers, which flow again into the sea. In our climate, evaporation is found to be about four times as much from the vernal to the autumnal equinox, as from the autumnal to the vernal. Heat facilitates all solutions; and the greater the difference between the temperature of the air and the evaporating surface, the greater will be the evaporation. Bishop Watson found that, even when there had been no rain for a considerable time, and the earth had been dried by the parching heat of summer, an acre of ground dispersed into the air above 1,600 gallons of water in the space of twelve hours of a summer’s day. A little reflection would convince any one of the importance of the principle of evaporation. Innumerable instances of its use might be adduced; suffice to add, that without it neither grass nor corn could be sufficiently void of moisture to lay up for use. Our clothes when washed could not be dried; neither could a variety of the most common operations, which conduce much to our comfort and convenience, be performed without it.