CLOUDS, RAINS, AND RIVERS
Every occurrence in Nature is preceded by other occurrences which are its causes, and succeeded by others which are its effects. The human mind is not satisfied with observing and studying any natural occurrence alone, but takes pleasure in connecting every natural fact with what has gone before it, and with what is to come after it. Thus, when we enter upon the study of rivers, our interest will be greatly increased by taking into account, not only their actual appearances but also their causes and effects.
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 Rhine and the Rhone in the Alps; 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. You may, however, 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 cannot end here. Whence comes the rain which 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 vapour 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 vapour mingles with the cold air above the hot funnel, it ceases to be vapour. 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 upon the character of the day. In humid 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 vapour.
The drier the air, and the hotter the air, the greater is the amount of cloud which can be thus dissolved in it. When the cloud first forms, its quantity is far greater than the air is able to maintain in an invisible state. But, as the cloud mixes gradually with a larger mass of air, it is more and more dissolved, and finally passes altogether from the condition of a finely-divided liquid into that of transparent vapour or gas.
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 issuing 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.
When the sunbeams fall upon the earth, they heat it, and also the water which lies on its surface, whether it be in large bodies, such as seas or rivers, or in the form of moisture. The water being thus warmed, a part of it is given off in the form of aqueous vapour, just as invisible vapour passes off from a boiler when the water in it is heated by fire. This vapour mingles with the air in contact with the earth. The vapour-charged air, being heated by the warm earth, expands, becomes lighter, and rises. It expands also, as it rises, because the pressure of the air above it becomes less and less with the height it attains. But an expanding body always becomes colder as the result of its expansion. Thus the vapour-laden air is chilled by its expansion. It is also chilled by coming in contact with the colder, higher air. The consequence is that the invisible vapour which it contains is chilled, and forms into tiny water-drops, like the steam from a kettle or the funnel of the locomotive. And so, as the air rises and becomes colder, the vapour gathers into visible masses, which we call clouds.
This ascending moist air might become chilled, too, by meeting with a current of cold, dry air, and then clouds would be formed; and should this chilling process continue in either case until the water-drops become heavier than the surrounding air, they would fall to the earth as raindrops. Rain is, therefore, but a further stage in the condensation of aqueous vapour caused by the chilling of the air.
Mountains also assist in the formation of clouds. When a wind laden with moisture strikes against a mountain, it is tilted and flows up its side. The air expands as it rises, the vapour is chilled and becomes visible in the form of clouds, and if sufficiently chilled, it comes down to the earth in the form of rain, hail, or snow.
Thus, by tracing a river backwards, from its end to its real beginning, we come at length to the sun; for it is the sun that produces aqueous vapour, from which, as we have seen, clouds are formed, and it is from clouds that water falls to the earth to become the sources of rivers.
There are, however, rivers which have sources somewhat different from those just mentioned. They do not begin by driblets on a hillside, nor can they be traced to a spring. Go, for example, to the mouth of the river Rhone, and trace it backwards. You come at length to the Lake of Geneva, from which the river rushes, and which you might be disposed to regard as the source of the Rhone. But go to the head of the lake, and you find that the Rhone there enters it; that the lake is, in fact, an expansion of the river. Follow this upwards; you find it joined by smaller rivers from the mountains right and left. Pass these, and push your journey higher still. You come at length to a huge mass of ice—the end of a glacier—which fills the Rhone valley, and from the bottom of the glacier the river rushes. In the glacier of the Rhone you thus find the source of the river Rhone.
But whence come the glaciers? Wherever lofty mountains, like the Alps, rise into the high parts of the atmosphere where the temperature is below the freezing-point, the vapour condensed from the air falls upon them, not as rain, but as snow. In such high mountainous regions, the heat of the summer melts the snow from the lower hills, but the higher parts remain covered, for the heat cannot melt all the snow which falls there in a year. When a considerable depth of snow has accumulated, the pressure upon the lower layers squeezes them into a firm mass, and after a time the snow begins to slide down the slope of the mountain. It passes downward from one slope to another, joined continually by other sliding masses from neighbouring 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 called a glacier.
Without solar fire, therefore, we could have no atmospheric vapour, without vapour no clouds, without clouds no snow, and without snow no glaciers. Curious then as the conclusion may be, the cold ice of the Alps has its origin in this heat of the sun.
Tyndall: "The Forms of Water."
(Adapted)
For what are men better than sheep or goats
That nourish a blind life within the brain,
If, knowing God, they lift not hands of prayer
Both for themselves and those who call them friend?
Tennyson