The water is, in some respects, a form between the other two; its peculiarities, weight, density, freedom of movement, and changeableness of form, are a mean between the opposing extremes of air and the ground. Water can pass to a more fluid or a more solid state; it can become vapor or ice. The measurement of the depth of the world of waters has lately been so clearly connected with the needs of civilization, that geographers have made many exceedingly accurate investigations. Formerly, this was much neglected; up to Captain Cook’s time 1500 feet was the greatest depth ascertained; in the course of the Arctic discoveries 7000 feet limited the plummet’s descent; Captain Ross sounded, near St. Helena, to a depth of 30,000 feet; and Captain Denhorn, in the South Atlantic, reached a point 46,000 feet from the surface—about twice the height of the loftiest mountains. And not single points alone, but entire ocean districts have been traversed; the temperature of these great depths has been studied, the currents, the density, in fact all the features which must be known preliminarily to the laying of great lines of submarine telegraphs, such, for example, as that proposed between North America and Europe. The atmosphere, too, is by no means thoroughly known to us. It rises to a height between 85 and 95 miles from the earth, of which man has explored in balloons only about five miles, or the height of the loftiest mountains. At loftier heights than we can live, the bright light of mid-day even fades into a dim kind of twilight, and meteoric masses of iron are seen in full glow, there being oxygen enough even there to support their combustion, and very little resistance to overcome from the density of the atmosphere. Astronomers, Benzenberg in particular, have calculated the distance of the meteors to range from 23 to 100 miles from the earth, and have studied them[2] in respect to the time when they were visible, their locality, and their direction. The limits of the atmosphere must be at that point where the expansive power of air and the attractive influence of the globe neutralize each other. The form of the atmospheric body is therefore, like the earth, spheroidal, but far more oblate than the earth, in consequence of its much greater fluidity. At the poles, the distance is therefore much less from the surface of the earth to the confines of the atmosphere than at the equator. The effect of this upon the refraction of light must be very great.