Besides supplying the vital necessity of water, rivers are, in many respects, an inestimable blessing to the countries through which they flow. Wherever they appear, the land is like a garden; where they are absent, it is a desert. Refer, for example, to the map of Egypt, and behold the Nile, its fruitful banks studded with innumerable towns, pursue its majestic and fertilising course. Beyond the sphere of its beneficial influence, the country on the east and on the west, exposed to the action of a burning sun, and unrefreshed by the moisture of a single stream, presents the aspect of an arid and uninhabitable wilderness. The advantages of rivers, in respect of their influence upon the soil, and in imparting coolness to the surrounding atmosphere, is thus most apparent in tropical countries. Under the combined operation of heat and moisture, vegetation in these regions reaches the maximum of luxuriance. "The Ganges," says Macaulay, "rushing through a hundred channels to the sea, has formed a vast plain of rich mould, which, even under the tropi cal sky, rivals the verdure of our English April. The rice fields yield an increase such as is elsewhere unknown. Spices and sugars, vegetable oils, are produced with marvellous exuberance."

Rivers, by facilitating intercourse between the different countries of the world, and thus materially contributing to the progress of commerce, have been highly conducive to civilisation and to the wealth and prosperity of nations. Most of the great commercial cities of the world, it will be found, are mainly indebted for their importance to a favourable situation on the bank of some navigable river, such as the Thames, the Elbe, or the Neva. Viewed as a means of transit, in a more limited sense, the value of rivers is not to be estimated. The immense quantities of timber, for example, that are annually floated down the Rhine, the Mississippi, and other streams, would, without such means of conveyance, be totally unavailable for the various purposes to which that useful material is so ex

tensively applied. The Rhine rafts, which are constructed upon an immense scale, deserve a passing notice. The wood that is floated down its tributaries, being collected at a considerable distance from its mouths, is formed into an immense raft, sometimes 800 or 900 feet in length, and 60 or 70 in breadth. A number of huts are erected on it for the accommodation of the workmen and rowers, amounting to several hundreds, so that the enormous mass has the appearance of a floating village.

The natural advantages of rivers as a medium of communication have been multiplied by human industry in the cutting of canals. There are upwards of one hundred canals in England; and they cover the surface of Holland like network.

The moral and physical aspect of Africa, which is comparatively destitute of rivers, affords a striking instance of their beneficial influence. In the absence of these vivifying agents, a great part of its surface is condemned to perpetual sterility; while, for the same reason, the interior being in a great measure excluded from communication with civilised nations, its inhabitants are consequently in a state of ignorance and barbarity.

It is one of the remarkable instances of God's goodness to man, that the enjoyments to be derived from His innumerable gifts do not depend on their usefulness alone. Thus, in contemplating the life and beauty that a noble river imparts to the landscape, we are filled with a sentiment of pleasure entirely irrespective of its mere utility; and the sense of its importance to our physical wants is lost in the more exquisite gratifications that pertain to the feelings. As the river is not in this respect a solitary example of the Divine benevolence, we trust that the consideration of such a subject may always be accompanied by those emotions of reverence and gratitude which it is so well fitted to excite

1. Write a short explanation of the following Properties of Matter.

MODEL.

Impenetrability.

Matter is distinguished by certain properties which are termed essential or general. The essential properties-that is to say, those without which we cannot conceive matter to exist―are Impenetrability, Extension, and Figure.

Impenetrability is that property by which every body occupies a certain space, so that when two bodies are pressed together, they are not lost in each other, however great the force employed. In the case of two solids, this principle is so apparent as to require no demonstration; while its effect in every other instance, though not so easily perceptible, is equally certain. Thus, before a vessel can be filled with water, the air that is contained in the vessel must be displaced, as we may prove by a very simple experiment. Let a glass vessel be forced under water with its mouth downwards, and it will be found that the water will not rise to the top until the air that is contained in the vessel is allowed to escape. It is upon this principle that the use of the diving-bell depends. Again, if we drop a stone into a vessel filled with water, as much of the water will run over as is equal to the bulk of

the stone.

From these and other instances we find that solids, liquids, and aëriform bodies are impenetrable; that is to say, it is impossible for two solids, for a solid and a liquid, for two liquids, or for a liquid and air, to occupy the same space at the same instant of time.

There are cases, however, in which a condensation takes place, when two solids, two liquids, or two gases, are mixed together, so that less space is occupied by the two united than was occupied by them separately. For example, an alloy of copper and tin contracts a fifteenth part of their former bulk; and a mixture of water and alcohol is attended by a similar result. We are not, however, to infer from these instances, that there are any cases in which two bodies or portions of two bodies can occupy the same space at the same time, the diminution in bulk being thus explained :-Every body in nature is composed of an infinite number of small particles, at a certain distance from each other, so that, when a mixture is made between two substances, the particles of the one sometimes interpose themselves between the particles of the other; and thus two bodies, when mixed together, may occupy less space than they did before the mixture took place.

2. Write a short explanation of the following Natural Phenomena.

MODEL.-The Tides.

The tides of the ocean, one of the most striking of natural phenomena, are caused by the attractive power of the sun and moon upon its waters.

Let us, for the sake of illustration, suppose that the whole surface of the earth were covered with water,

and that no external influence were brought into operation. In that case, the water would necessarily remain everywhere at a uniform level. Let us bear in mind, however, that, by the universal law of gravitation, all bodies attract each other in a degree regulated by the distance between them, and we shall then see what effect such a body as the moon, situated as she is in respect to the earth, must have upon this supposed uniformity of level. While she absolutely attracts the whole mass of the earth, the waters lying immediately under her, being operated upon with greater intensity than those that are more remote, rise in a heap, and thus form what is termed high water, or full tide. Again, the waters at the corresponding point of the opposite hemisphere, being evidently that part of the whole mass which is least subject to her attractive influence, are consequently drawn forward with less energy than the intervening portions; and, being thus left behind, accumulate in such a manner as to form a tide at the same time as that produced by the direct attraction of the moon. It is evident that low water must lie on both sides of the earth between the two elevations. It is also evident that we must have two tides in the course of the earth's revolution round its axis. This is accomplished in twenty-four hours; but, as the moon is in the meantime proceeding in her course, it is nearly an hour more before the earth returns to the same relative position. We have thus two tides in about twenty-five hours.

The sun, on account of its immense distance, affects the tides less than the moon does. When these two bodies attract the waters in unison, as at new and full moon, the tides are highest, and are thence named spring tides. When, on the other hand, their attraction is in opposition, that is to say, when the moon is in her first and third quarters, the tides are lowest, and are therefore denominated neap tides.