In consequence of the effects of mountains in cooling currents of moist air, and causing the condensation of aqueous vapor in the manner above described, it follows that in every country, as a general rule, the more elevated regions become perpetual reservoirs of water, which descends and irrigates the lower valleys and plains. The largest quantity of water is first carried to the highest region, and then made to descend by steep declivities towards the sea; so that it acquires superior velocity, and removes more soil, than it would do if the rain had been distributed over the plains and mountains equally in proportion to their relative areas. The water is also made by these means to pass over the greatest distances before it can regain the sea. It has already been observed that in higher latitudes, where the atmosphere being colder is capable of holding less water in suspension, a diminished fall of rain takes place. Thus at St. Petersburg, the amount is only 16 inches, and at Uleaborg in the Gulf of Bothnia (N. lat. 65°), only 13½ inches, or less than half the average of England, and even this small quantity descends more slowly in the temperate zone, and is spread more equally over the year than in tropical climates. But in reference to geological changes, frost in the colder latitude acts as a compensating power in the disintegration of rocks, and the transportation of stones to lower levels. Water when converted into ice augments in bulk more than one-twentieth of its volume, and owing to this property it widens the minute crevices (or _joints_) of rocks into which it penetrates. Ice also in various ways, as will be shown in the next chapter, gives buoyancy to mud and sand, even to huge blocks of stone, enabling rivers of moderate size and velocity to carry them to a great distance. The mechanical force exerted by running water in undermining cliffs, and rounding off the angles of hard rock, is mainly due to the intermixture of foreign ingredients. Sand and pebbles, when hurried along by the violence of the stream, are thrown against every obstacle lying in their way, and thus a power of attrition is acquired, capable of wearing through the hardest siliceous stones, on which water alone could make no impression. _Newly formed valleys._--When travelling in Georgia and Alabama, in 1846, I saw in both those States the commencement of hundreds of valleys in places where the native forest had recently been removed. One of these newly formed gulleys or ravines is represented in the annexed woodcut (fig. 14), from a drawing which I made on the spot. It occurs three miles and a half due west of Milledgeville, the capital of Georgia, and is situated on the farm of Pomona, on the direct road to Macon.[268]