PHYSICAL GEOGRAPHY.

Turn to the little streamlet or drain a few yards off, that is still running rapidly, carrying away the rain water that has fallen in the morning, and which is a great deal dirtier than that of the river. You know that when the rain fell, the water was pure and clean, for perhaps you may have had your clothes wetted by it, but it only wetted, and did not dirty them. Clearly then the mud and dirt with which the water of that little drain is so highly charged has been carried away from the land; and the same thing is true of the swollen river. All that great body of water was rain a few hours, days, or weeks ago, and having fallen on the ground, has gradually collected in little channels and drains like that beside you; and as these joined together, they formed at first small streams, then larger streams, and lastly a river. All the mud and dirt that it contains has been washed off the land, and is being carried down to the sea, and there are no rivers running back from the sea to bring it back to the land. Is the land then always being washed away into the sea? Yes, at all times. Every river is doing its work of destruction; but it is not all destruction. You may learn from the cultivators about you, that some neighbouring jhíl is being gradually filled up by the earth settling down from the water that overflows into it from the river at each flood; and year by year, some little piece of it that was formerly too deep under water to be cultivated, is reclaimed and added to the rice lands around its margin. We shall see presently that the whole of the broad rice-fields of lower Bengal have been actually formed in this way. It is a very slow process. It must have taken many thousands of years for all Bengal to be formed by the mud settling from the rivers; but we shall find presently that the ground contains the evidence of its own history, a history which we may read by simply continuing to observe, and to reason on what we observe, in the way that I have now shown you; and thus we learn that all the great plains of Bengal must at one time have been occupied by the sea, and that this sea has been gradually filled up by the sand and mud brought down by the rivers.

We will now leave for a time this story of the making and destruction of land by the river, and will turn our attention to the water. What makes the river flow always one way, and why is it so full at a particular season of the year, and low at other times? We shall find the answers to these questions not less interesting than that which we have already arrived at. In the first place, a very little observation will show you that water will not run up-hill. If a cultivator wants to water his crops from some neighbouring pool or well, he must make a channel that is a little higher at the end where the water is poured into it than where it delivers it on his field; and by means of a paicottah, or a scoop, he must raise the water from the pool, and pour it into the higher end of the channel. The more his channel slopes, the faster does the water run down it. Now what is true of his little water-channel is equally true of the river. To your eye the surface of the river appears level, but that is only because it slopes so little, that your eye cannot distinguish the slope from the real level of a jhíl. If you go down the Hooghly in a boat from Barhampúr, or Murshidabád, to Sagar Point, at every part of your journey the watersurface will seem to be as level as that of the river before you; but we know from measurements very carefully made by surveyors, that in this journey you have really gone down a slope, such, that at the end of it, you would be about 60 feet lower than when you started. În like manner if you were to start in a boat from Benares to travel to Dacca, the water would seem to be quite level from first to last; but actual measurement shows that you would have descended about 240 feet; and had you ended your journey at Monghyr, you would then have descended more than 100 feet without being in the least aware of it.

The river flows then always in the same direction because it flows down a slope; but the slope is so gentle that it is necessary to use a very delicate instrument termed a level in order to measure the fall. The force that makes the water flow is the same that makes a stone fall back to the ground when you throw it into the air, and is called

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gravitation. We shall have much to say of this force presently.

If you have clearly understood what I have said so far, your own reason will tell you that the water must be raised by some means or other to the place where the river begins. To explain how this is accomplished, we will have recourse to another very simple experiment. Take a shallow dish which must be such that water cannot soak into it. Fill it with water and let it stand in the open air, where it will not be disturbed. If the weather is fine it will slowly disappear; and in the course of a day or two it will be all gone, and the dish will be dry. It has all gone into the air; you cannot see it because it has become a part of the air, and has assumed the state which is termed vapour. The air always contains some of this vapour, which it gathers from the surface of the sea, the rivers, and indeed wherever there is water. Most of it comes from the sea, which is always evaporating or giving off vapour under the heat of the sun; and this vapour is carried by the winds to places, a long way from where it was produced. Much of it is carried to the land, and sometimes, as between June and September, when the wind blows steadily and strongly from the sea, it is carried far into the interior. But it does not always remain vapour. You may often observe, a few hours after sunrise, when the sky has been quite clear from cloud during the morning, that clouds with rounded tops and flat below gradually make their appearance, not visibly coming from anywhere, but forming high up in the air. Now these clouds are the vapour of the air again converted into water, and although they sometimes disappear again by re-evaporating as the day wears on, at other times they get gradually thicker and darker, and at last send down a shower of rain. It is then from the vapour which is carried in the air that rain is formed, and we have already seen that it is the rain water that collects to form rivers.

Let us now stop for a moment, and recapitulate what we have learned. The sea furnishes vapour to the air, just as, on a small scale, a dish of water will do if left standing.

The vapour is carried away by the winds, which are air in motion; and being reconverted into water in very minute drops, forms clouds: the clouds, when very thick, furnish the rain, which is merely those very minute drops uniting together to form larger drops: the rain runs off the land, carrying with it particles of mud and sand, and then collects in streams and rivers, which are turbid with the materials brought into them by the drainage water. These flow down the sloping surface of their beds into the sea, constantly carrying thither portions of the solid land, which are never brought back with the evaporated water. At the same time when rivers overflow their banks, and fill jhíls and places where the water is still, some of this earth settles down and forms new land; but, after all, this is very little in comparison with that which is carried away. The rest sinks gradually to the bottom of the sea, and if there were no new land formed, by some power which we have not yet spoken of, in the course of time all the land would be carried away; first the mountains and hills, which are wasted more rapidly than the plains, and finally the plains themselves. But there is such a power, as we shall find out somewhat later on, though its action is not so readily to be observed as the familiar changes produced by the rain and the rivers; and it will be better to defer its consideration until we have made some further progress in our knowledge of more familiar things.

You will now be prepared to give a partial answer to the question, Why is the river so full during a part of the year, and so low at other times ? You know that its fulness depends on the quantity of rain that falls, and that rain falls much more abundantly from June to September than during the remainder of the year. But this answer only suggests other questions. For we have to ask the very similar question, why does the rain fall more abundantly at one season than another? and if we further reply, because the wind then blows from the sea, which furnishes abundance of vapour, while during the dry cold weather it blows from the land where there is very little water to furnish vapour, both

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of them facts that a little observation will show us to be true, the change of the winds still remains for explanation. This too may be explained as we shall see, but not in a few words, nor without a great deal more observation than we have yet given to what is going on around us.

Now observe what a number of interesting enquiries have been suggested by a few observations thoughtfully made, on things so familiar to you, that perhaps you have never yet thought there is anything to be learned from them. They have led us on to the strange and unexpected conclusion that the very land on which we live is gradually wasting away, and since there is so much land still left to live on, it seems probable that there must be some means by which new land is formed; but what those means are we have not yet learned. We have learned, too, that even the variable winds show some regularity, since they blow from one quarter during a part of the year, and from another quarter at another season, but we have yet to learn what is the cause of this regular change; and we have also to learn how clouds are formed from invisible vapour.

But the most important lesson that you will have gained, if you have, as I suppose, really seen and thought over what I have described, and have not been idly contented with reading what I have written, will be this: that the best of all kinds of knowledge is to be gained, not from what is written in books, but by watching what takes place around you, by thinking over what you see, and whenever you can, resorting to experiment, to decide something or other which observation alone has not sufficed to explain. All the knowledge we have of nature has been gained by these two methods, observation and experiment; and the chief use of books is to tell you of what you cannot see for yourself, but which others have seen, and to show you how others have thought and enquired about these things, so that you may follow their example.

You will now have gained some idea of what is meant by Physical Geography. It is the story of the changes that go on, and are always going on around us in this world of ours.