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. In such a country as ours, where there is no perpetual snow, you can without much difficulty answer this question. Each fall of snow in winter-time remains on the ground as long as the air is not warm enough to melt it. Evaporation, indeed, goes on from the surface of snow and ice, as well as from water: so that a layer of snow would in the end disappear, by being absorbed into the air as vapor, even though none of it had previously been melted into running water. But it is by what we call a thaw that our snow is chiefly dissipated; that is, a rise in the temperature, and a consequent melting of the snow. When the snow melts, it sinks into the soil and flows off into brooks in the same way as rain. In the regions of perpetual snow the heat of summer can not melt all the snow which falls there in the year. What other way of escape, then, can the frozen moisture find? You will remember that the surplus rainfall flows off by means of rivers. Now the surplus snow-fall above the snow-line has a similar kind of drainage. It flows off by means of what are called glaciers. When a considerable depth of snow has accumulated, the pressure upon the lower layers from what lies above them squeezes them into a firm mass. The surface of the ground is usually sloped in some direction, seldom quite flat. And among the high mountains the slopes are often, as you know, very steep. When snow gathers deeply on sloping ground, there comes a time when the force of gravity overcomes the tendency of the pressed snow to remain where it is, and then the snow begins to slide slowly down the slope. From one slope it passes on downward to the next, joined continually by other sliding masses from neighboring slopes until they all unite into one long tongue which creeps slowly down some valley to a point where it melts. This tongue from the snow-fields is the glacier. It really drains these snow-fields of their excess of snow as much as a river drains a district of its excess of water.