the sea is about five miles, and it is probable that the greatest depth of the sea is not much more. Now the earth is a globe, the diameter of which is sixteen hundred times as great as this, so that the utmost depth of the sea, on an artificial globe sixteen inches in diameter, would be represented by a thin fibre only a hundredth part of an inch thick, or about as thick as the paper on which this is printed.

Still, wherever there is an ocean of considerable extent, measuring from east to west, there will be found a tide-wave on the same principles as we have already supposed, the ridge of which follows the apparent course of the moon from east to west. Now, the only part of the sea in which the action of the moon upon the waters can cause anything like such a regular tide is the Great Southern ocean, including the southern part of the Atlantic and Pacific Oceans, and of the Indian Sea. Although this great belt of water does not lie under the Equator, it extends with little interruption, in a direction from east to west, round the whole of the globe. In these seas, then, we may look for a tide of great regularity; and it is accordingly found.

The sea next in extent, in a direction from east to west, is the remaining part of the Pacific Ocean.

With respect to the Atlantic Ocean, although it extends nearly from Pole to Pole, in a direction from north to south, its breadth from east to west is by no means so great; and for the present purpose we may consider it as a great arm of the Southern Ocean, stretching in a direction at right angles to the course of the general tide-wave in that open sea.

To understand how the tides in such an arm of the sea are formed, let us suppose a long trough, P Q, and a narrower trough, C K, opening into it. Now, suppose the water in PQ to be set in motion so as to have a succession of waves passing along from P to Q, and suppose and B to be two successive ridges of such waves, with a hollow between them at L. Then, when the ridge A is at C, the water will be highest at C; as the ridge moves along, the water at C will sink, and be the

lowest when L reaches C; and it will again rise until the second ridge B has reached C.

But it is plain that since there is nothing to stop some of the water of the ridge A from running along the trough C K, to find its level, part of it will run along and form a movable ridge (a), which will advance along C K exactly in the same manner as A moves along P Q. There will therefore be a new set of waves moving along C K, not in the direction of the width of C K, but in the direction of its length.

It must also be observed that the ridge (a) may not move so fast as the original ridge A, but that the time elapsed between the passage of two successive ridges past any point (as m, in C K) will be the same as the time between the passage of two successive ridges, A B, past; since the ridge B would give rise to a wave under the very same circumstances as those in which A caused one.

Now we may conceive P Q to represent the Great Southern Ocean, along which the tide wave is constantly passing, in the direction PQ, from east to west. In like manner, C K may represent the Atlantic Ocean, of which m is on the African coast, and n on the American coast. And we shall have a succession of tide-waves, such as (a,) moving from south to north, and succeeding ong another, after the same interval of time, as that in which A succeeds B, or a little more than twelve hours.

Accordingly, it is found that, in the Atlantic Ocean, the tide-wave does move from south to north, the ridge

of the waves extending in a slanting direction, and in an irregular form, across from the African to the American coast.

In order to explain the manner in which these waves cause the tide in different branches of the same sea, we will trace the course of the tide-wave round the coast of

Suppose the moon to have passed the meridian of Ushant, on the north-west part of the coast of France, at twelve o'clock in the day, the tide-wave of the Atlantic will reach Ushant soon after three o'clock on the same afternoon, its ridge stretching towards the north-west, so as to fall a little south of Cape Clear, in Ireland.

This wave soon after divides itself into three branches.

One part passes eastward up the English Channel, causing high-water in succession at all the places at which it arrives. It moves at about the rate of fifty miles an hour, so as to pass through the straits of Dover and reach the Nore about twelve o'clock at night. The second branch of the tide-wave passes more slowly up the Irish Channel, causing high-water along the coast of Wales, Lancashire, and Cumberland, and upon the eastern coast of Ireland. The third and principal part of the same wave moves more rapidly, being in a more open sea. By six o'clock it has reached the northern extremity of Ireland: about nine o'clock it has got to the Orkney Islands, and forms a wave extending due north. At twelve o'clock at night, the summit of the same wave extends from the coast of Buchan in Scotland, eastward to the Naze in Norway, and in twelve hours more it has flowed down the eastern coast of England, forming the flood-tide from the north, and reached the Nore, where it meets the morning tide which left the mouth of the English Channel above eight hours before.

The consequence of the meeting of the two tides at the Nore is very remarkable in the Thames. Sometimes the tide from the north is a little later than the other, and continues to flow after the other has ebbed considerably, thus causing a second tide on the same day. Another consequence is that, on the whole eastern coast of England, the tides are upon the whole highest when the wind blows strongly from the north-west, or off shore. This may appear strange at first, but the cause is quite plain when we remember that the tide is caused by such a wave as has been described, passing round the northern extremity of Scotland into the German Ocean.

It will be seen also that the tide in the English Channel is twelve hours earlier than the tide in the German Ocean: so that if the highest spring-tide from the south reached the Nore at twelve o'clock in the day, the highest spring-tide from the north would not occur till twelve o'clock at night.-S. M.

LESSONS IN GEOLOGY.

AROUND each village or town various soils are found; sometimes they resemble one another; sometimes they do not; sometimes expanses of chalk prevail; sometimes tracts of gravel; sometimes granite abounds; sometimes sandstone. Geology is the science which explains the differences between these various soils. It takes into consideration the structure of the entire earth, analyzes that structure, arranges its various parts, and seeks to explain the origin of those parts, and the reason of their present arrangement. But Geology has practical as well as theoretical uses; and the former render its study important and necessary to the miner, the railway constructor, the architect, and the builder.

The means of practically studying this science exist within the reach of all. A heap of stones by the roadside unfolds to one acquainted with Geology the history of past ages. Every quarry, every railway cutting, every river-bank, every sea-side cliff, every well sunk, all shew the successive stages through which the earth has passed previous to attaining its present state and condition.

The geologist explains the earth's past history by means of the present operations of nature. He sees layer of earth overlying layer of earth, and he knows, from observation, that those layers have received their present position from being deposited by water. He observes how some rocks appear twisted and distorted, and he explains it by the pressure, both lateral and perpendicular, of older and harder rocks. He watches the effect of the atmosphere upon rocks, and thus accounts for their wasted appearance, and for the immense boulders precipitated from rock-summits to the valleys beneath. He stands by the river-side, and notices that the banks are gradually being washed away. observes the mud that discolours the waters, and by its means he can account for the delta that the river forms at its mouth. He travels into distant lands, and the violent action which accompanies the cruptions of

He