The Chautauquan, Vol. 04, December 1883

by Chautauqua Literary and Scientific Circle; Chautauqua Institution

When the current of a river is checked as it enters the sea or a lake, the feebler flow of the water allows the sand and mud to sink to the bottom. By degrees some portions of the bottom come in this way to be filled up to the surface of the river, and wide flat marshy spaces are formed on either side of the main stream. During floods these spaces are overflowed with muddy water, in the same way as in the case of the valley plains just described, and a coating of mud or sand is laid down on them until they slowly rise above the ordinary level of the river, which winds about among them in endless branching streams. Vegetation springs up on these flat swampy lands; animals, too, find food and shelter there; and thus a new territory is made by the work of the river.

These flat river-formed tracts are called deltas, because the one which was best known to the ancients, that of the Nile, had the shape of the Greek letter Δ (delta). This is the general form which is taken by accumulations at the mouths of rivers; the flat delta gets narrow toward the inland, and broader toward the sea. Some of them are of enormous size; the delta of the Mississippi, for example.

Each delta, then, is made of materials worn from the surface of the land, and brought down by the river. And yet vast though some of these deltas are, they do not show all the materials which have been so worn away. A great deal is carried far out and deposited on the sea-bottom; for the sea is the great basin into which the spoils of the land are continually borne.

Having now followed the course taken by the water which falls on the land as rain, we come to that taken by snow.

On the tops of some of the highest mountains in Britain snow lies for great part of the year. On some of them, indeed, there are shady clefts wherein you may meet with deep snow-wreaths even in the heat of summer.

But in other parts of Europe, where the mountains are more lofty, the peaks and higher shoulders of the hills gleam white all the year with unmelted snow.

Let us see why it is that perpetual snow should occur in such regions, and what part this snow plays in the general machinery of the world.

You have learned that the higher parts of the atmosphere are extremely cold. You know also that in the far north and the far south, around those two opposite parts of the earth’s surface called the Poles, the climate is extremely cold—so cold as to give rise to dreary expanses of ice and snow, where sea and land are frozen, and where the heat of summer is not enough to thaw all the ice and drive away all the snow. Between these two polar tracts of cold, wherever mountains are lofty enough to get into the high parts of the atmosphere where the temperature is usually below the freezing-point, the vapor condensed from the air falls upon them, not as rain, but as snow. Their heads and upper heights are thus covered with perpetual snow. In such high mountainous regions the heat of the summer always melts the snow from the lower hills, though it leaves the higher parts still covered. From year to year it is noticed that there is a line or limit below which the ground gets freed of its snow, and above which the snow remains. This limit is called the snow-line, or the limit of perpetual snow. Its height varies in different parts of the world. It is highest in the warmer regions on either side of the equator, where it reaches to 15,000 feet above the sea. In the cold polar tracts, on the other hand, it approaches the sea-level. In other words, while in the polar tracts the climate is so cold that perpetual snow is found even close to the sea-level, the equatorial regions are so warm that you must climb many thousand feet before you can reach the cold layers of the air where snow can remain all the year.

There is, you see, one striking difference between rain and snow. If rain had been falling for the same length of time, the roads and fields would still have been visible, for each drop of rain, instead of remaining where it fell, would either have sunk into the soil, or have flowed off into the nearest brook. But each snowflake, on the contrary, lies where it falls, unless it happens to be caught up and driven on by the wind to some other spot where it can finally rest. Rain disappears from the ground as soon as it can; snow stays still as long as it can.

You will see at once that this marked difference of behavior must give rise to some equally strong differences in the further procedure of these two kinds of moisture. You have followed the progress of the rain; now let us try to find out what becomes of the snow.