Let us trace a river to its source. Beginning where it empties itself into the sea, and following it backwards, we find it from time to time joined by tributaries which swell its waters. The river of course becomes smaller as these tributaries are passed. It shrinks first to a brook, then to a stream; this again divides itself into a number of smaller streamlets, ending in mere threads of water. These constitute the source of the river, and are usually found among hills.

Thus, the Severn has its source in the Welsh mountains; the Thames in the Cotswold Hills; the Missouri in the Rocky Mountains; and the Amazon in the Andes of Peru.

But it is quite plain that we have not yet reached the real beginning of the rivers. Whence do the earliest streams derive their water? A brief residence among the mountains would prove to you that they are fed by rains. In dry weather you would find the streams feeble, sometimes, indeed, quite dried up. In wet weather you would see them foaming torrents. In general these streams lose themselves as little threads of water upon the hillsides; but sometimes you may trace a river to a definite spring. But you very soon assure yourself that such springs are also fed by rain, which has percolated through the rocks or soil, and which, through some orifice that it has found or formed, comes to the light of day.

But we can not end here. Whence comes the rain that forms the mountain streams? Observation enables you to answer the question. Rain does not come from a clear sky. It comes from clouds.

But what are clouds? Is there nothing you are acquainted with which they resemble? You discover at once a likeness between them and the condensed steam of a locomotive. At every puff of the engine a cloud is projected into the air.

Watch the cloud sharply. You notice that it first forms at a little distance from the top of the funnel. Give close attention and you will sometimes see a perfectly clear space between the funnel and the cloud. Through that clear space the thing which makes the cloud must pass. What then is this thing which at one moment is transparent and invisible, and at the next moment visible as a dense opaque cloud?

It is the steam or vapor of water from the boiler. Within the boiler this steam is transparent and invisible; but to keep it in this invisible state a heat would be required as great as that within the boiler. When the vapor mingles with the cold air above the hot funnel, it ceases to be vapor. Every bit of steam shrinks, when chilled, to a much more minute particle of water. The liquid particles thus produced form a kind of water dust of exceeding fineness, which floats in the air, and is called a cloud. Watch the cloud banner from the funnel of a running locomotive: you see it growing gradually less dense. It finally melts away altogether, and, if you continue your observations, you will not fail to notice that the speed of its disappearance depends on the character of the day. In moist weather the cloud hangs long and lazily in the air; in dry weather it is rapidly licked up. What has become of it? It has been reconverted into true invisible vapor. The drier the air, and the hotter the air, the greater is the amount of cloud which can be thus dissolved in it.

Make the lid of a kettle air-tight, and permit the steam to issue from the spout; a cloud is formed in all respects similar to that which issues from the funnel of the locomotive. To produce the cloud, in the case of the locomotive and the kettle, heat is necessary. By heating the water we first convert it into steam, and then by chilling the steam we convert it into cloud. Is there any fire in nature which produces the clouds of our atmosphere? There is—the fire of the sun.

By tracing the course of a river, we find that both its beginning and its ending are in the sea. All its water is derived from the sea, and to the sea it returns its floods. But if we seek for its causes, we find that its beginning and its ending are in the sun. For it is the fire of the sun that produces the clouds from which the water of the river is derived, and it is the same fire of the sun that dries up its stream.