It is through the same relations of water to heat, that springs are supplied--for these undoubtedly draw their principal supplies from rain. Mr. Dalton has calculated that the quantity of rain which falls in England is 36 inches a year. Of this he reckoned that 13 inches flow off to the sea by the rivers, and that the remaining 23 inches are raised again from the ground by evaporation. The 13 inches of water are of course supplied by evaporation from the sea, and are carried back to the land through the atmosphere. Vapor is perpetually rising from the ocean, and is condensed by cold in the hills and high lands, as is easily recognized by the mists and rains, which are frequent in such regions; whence it descends through their pores and crevices, till it is deflected, collected and conducted out to the sea, by some stratum or channel which is water-tight, thus keeping up a perpetual and compound circulation. In every country these two portions of the aqueous circulation have their regular and nearly constant proportion; and their due distribution appears to be necessary to its organic health, to the habits of vegetables and of man. This circulation goes on from year to year as regularly as that of the blood, in the veins and arteries of the human system, and though maintained by a very different machinery, is no less clearly adapted to its purposes. In short the properties of water which regard heat make one vast watering engine of the atmosphere, (_Whewell_.) COMMON WATER. Under this head are included the waters commonly known as _rain_, _spring_, _river_, _well_ or _pump_, _lake_ and _marsh waters_. Thomson includes _ice_, and _snow water_, _spring_ and _river water_, and _lake water_ under _rain water_, as it is from this source that they are chiefly supplied. RAIN WATER is the purest kind of all natural waters, though subject to some variations. Thus, when collected in large towns or cities, it is less pure than when obtained in the country; moreover it is usually loaded with impurities at the commencement of a shower, but after some hours of continuous rain it becomes nearly pure; for the first water which falls brings down the various foreign matters suspended in the atmosphere. In specific gravity, it scarcely differs from distilled water. It nevertheless generally holds in solution common air, carbonic acid, carbonate of lime, chloride of lime, and a trace of nitric acid. If it be collected from the roofs of houses, after it has rained for some time, it contains sulphate of lime and occasionally carbonate of lead. The quantity of common air in rain water does not exceed 3½ cubic inches in 100 cubic inches of water; it contains more oxygen than atmospherical air; the same quantity of rain water contains one inch of carbonic acid gas.