lies between the capes, at the mouth of the Delaware, and the city of Philadelphia, forms a long wave, the distance from ridge to ridge being one hundred and forty miles: when it is high water at Philadelphia and at the mouth, the wave has the position represented in fig. 1, in which P represents Philadelphia and C the

P

M

Fig. 1.

capes: and when it is low water at the same points, the surface has assumed the position represented in fig. 2: the water having sunk at the two extremities, and risen in the middle.

In rivers of very great length there may be several of these tide-waves going on at once, causing high water

P

M

Fig. 2.

at every ridge, and low water at every hollow; and producing the different variations of the tide at the corresponding points of each wave, in the manner represented in fig. 3.

It is therefore a great mistake to suppose that when it is high water, for instance, at London Bridge, the water is at the same level all the way down the river. The water will continue to rise at London Bridge some time after it has begun to sink at Gravesend, and again will be sinking at London Bridge for an hour after the water has begun to rise at Gravesend.

It will be seen, also, that although the water is much deeper at any place, at high water than at low water,

yet, in a whole river of great extent there may not be much more water at one time than at another; and that the currents caused by the tides will, upon the whole, act as much one way as another.

H

H

H

H

Fig. 3.

On the coast of Suffolk, near where an opening has been made into the sea, to form a canal which shall be navigable for ships to Norwich, a circumstance is said to occur which shews very clearly the motion of the

C

tide-wave up the channel of a river. Upon a great part of that coast the sea is constantly throwing up a shingly beach, which stops the straight course of the rivers into the sea, and causes them to run along within a few yards of the sea before they can find an outlet. Such a river runs near the coast at C, where its mouth originally was: but it is there turned to the southward by the high beach, and really enters the sea at M, some miles lower down. Now it is high water in the sea at A when the tide-wave, coming from the north, arrives there; it is high water at M somewhat later; but it is not high water at C, in the river, until the tide wave from M has been propagated along the narrow and winding bed of the river from M to C. It so happens, that nearly six hours are taken up in the progress of the tidewave from A round M to C; so that by the time it is high water at C in the river, it is low water in the sea at A, only a few yards distant; and, again, when it is low water at C, it is high water at A.

M

The height of the tides at different places depends upon the direction and form of the coast, and other

causes, which vary with almost every different situation. The highest tides upon the coast of England occur in the Severn, where the tide-wave comes in in one large ridge, accompanied with a roaring noise, and with such violence as often to prove destructive to the small craft. It rises there to the height of forty feet.

The reason of the height of this tide is easily seen. The mouth of the Bristol Channel is very wide, and opens to the south-west, so as to receive the tide-wave from the Atlantic Ocean; but the Channel becomes narrower by degrees, and near Chepstow is very much contracted; the water is, therefore, heaped up at the other end of the Channel, much above the level to which it would otherwise rise.-S, M.

DISTRIBUTION OF PLANTS AND ANIMALS.

How many different kinds of plants one sees in the fields! What endless variety exists! Each nook conceals from our view some stray plant, found only after close examination. And if so many are found in one field, what immense numbers there must be in Great Britain! And, what must be the number of varieties of trees and plants found throughout the world?

And yet all these apparently endless varieties are arranged with great taste and skill by the hand of Nature; and each plant and each tree can be assigned to its own class by the botanist. Every country possesses its own vegetation; every country produces that for which its climate is best fitted Walk through a botanical garden, and at once you will see the marked contrasts presented by the vegetation of different countries. Here are tall, towering palms from Africa, there are the sweet-smelling balsams of Arabia, here arc the long narrow-leaved plants of Australia, there is the

woody interlaced work of the climbing plants of South America, while in another part are the shady trees of our own dear England.

What causes these differences? Climate. Why do not all countries possess the same kind of vegetation? Because they do not all possess the same climate. No two places on the earth's surface possess exactly the same climate, and the further we proceed from the equator, or the higher we ascend mountains, the more do we find the face of nature to change. It is as though the earth assumed various dresses to suit her various climates.

Now, let us take a few of the terms used with regard to plants and animals. Plants and animals can be viewed as distributed either in horizontal or vertical space; the former refers to their distribution, at the sea-level, from the equator to the poles, the latter refers to their distribution up the sides of a mountain. The entire vegetation of a country is called its Flora. The entire group of animals peculiar to a country, is called its Fauna. The general aspect of a country's vegetation is called its facies, thus we may say that the facies of North American vegetation resembles that of Northern Europe. Plants which belong to similar classes, but have developed differently under the influences of climate, are said to be representative, while plants peculiar to certain countries are said to be characteristic. The same terms are applied to animals.

Man has effected many changes in the distribution of vegetable and animal life. He has removed and transplanted, to such an extent, that in many cases the old floras have ceased to exist. Australia has, for instance, a characteristic flora. Its vegetation resembles that of no other part of the world. When men first visited it, they said it was a land of anomalies, every thing grew the wrong way; the cherry grew with the stone outside; many trees grew with their roots in the air, ferns grew to the size of trees. But these stories proved untrue.

Those who brought them were deceived. The species of plants that they saw in Australia were totally different to the species they saw in England. They belonged to different classes. The hot, dry, parched soil of Australia could only produce the tall, thin shrivelled-up plants they had seen. Since then, Australia has been colonized; methods of irrigation have been introduced, and plants, similar to our own, now flourish.

The South American forests are rich in flowers of gorgeous colours, and thickly interwoven trees; the climbing plants form such a complete net-work, that in many parts the forests are impassable. In the tropical regions of Asia, vegetation is equally rich, but developes itself differently under different influences. There the banyan-tree flourishes, the camphor-tree, the cotton-tree, the rice plant, and all kinds of palms and tree ferns. The Tropical regions of Africa, on the other hand, are remarkable for the scantiness of their vegetation.

From America and Western Asia we, in Europe, have received the greater portion of our plants and trees, and yearly more plants are transplanted and acclimatized.

The colder the climate gets, the lowlier the species of vegetation that exists, until, on the shores of the Atlantic and on the tops of the highest mountains, the only life existing consists of lichens and mosses.

Many animals depend upon vegetation for their existence. Change the flora of a country, then, and you will at once change its fauna. Since the colonization of Australia the old animals have begun to die out. A few more years will see the extinction of the kangaroo, the dingo, and some other species of animals now found there.

Animals are also representative and characteristic. The three just mentioned are characteristic of Australia; they are found only in that continent and the islands immediately to the north of it. The lion of Asia differs from the lion of Africa, while the puma (a much smaller animal), represents the class in America. Tigers again