its diurnal revolution, the stars will return to the relative places they before occupied; and as the velocity of the earth's rotation is uniform, this will occur in equal times.

But let us suppose our observer to pursue his investigations a little more closely. He has already discovered that the stars apparently move in the arc of a circle, rising in the east and setting in the west. Towards the south there are some that just rise above the horizon, take a short arc, and disappear; while towards the north there are some that never set, but revolve in very small circles around a point, which is called the north pole; but this point is not marked by any star, although there is one so near to it, that the unassisted eye cannot discover its motion. As soon as the observer has ascertained these facts, he becomes conscious that there is one section of the celestial sphere that is hidden from his view; for as there is a segment containing stars which never set, so there must be one in which stars revolve without rising, and these are called the south polar stars. To obtain a view of these, he must travel southward. As he proceeds on his journey, the north polar stars will approach the horizon, and the stars of the southern hemisphere will be proportionally raised. When he has reached that line on the surface of the earth called the equator, that is, an imaginary line, dividing it into hemispheres, the poles will be in his horizon, and every star will appear to perform half its revolution above and half below his horizon, and no part of the heavens will be hidden from his view. And here we may suppose the inquiring traveller to be struck with the fact, that although the paths of the stars are so different in extent, those which rise exactly in the east having much the longest arcs, and those at the poles the shortest, yet every star is above the horizon for the same period of time. But if the observer still travel southward, the south pole of the heavens will be raised above his horizon, and the north pole will be depressed below it; and when he is as near to the south pole of the earth as he was to the north pole at the commencement of his journey, the phenomena will be entirely reversed; the south polar stars will never set, the north polar will never rise; and if he could still travel southward and reach the pole itself, the stars would appear to revolve in circles parallel to the plane of his horizon, and to one another.

These results cannot fail to confirm the conviction, that

the motion of the stars is only apparent, and that it is produ, ced by the revolution of the earth on its axis.

THEORY OF THE FORMATION OF THE EARTH FOUNDED ON ITS DIURNAL REVOLUTION.

Now it is a singular fact, that these results perfectly accord with the combined theoretical deductions of astrono, mers and geologists. The covering or crust of the earth is known to consist of a series of strata of different substances and of various thicknesses. Beds of clay and marl, limestone and sandstone, gravel and sand, are promiscuously mingled together, many of them containing the remains of marine and fresh-water animals, and all of them bearing evidence of their formation as resulting from the deposition of water. Some have been formed at the bottom of rivers and lakes, and some in seas and oceans, while others have been produced by casual catastrophes, which have caused the waters to leave their channels, and sweeping over localities, or the entire surface of the earth, to destroy rocks, and round the fragments they have broken from the parent bed. The same process of destruction and reproduction is going on in the present day, though not perhaps to so great an extent as at the time when the crust of the earth was formed. If the beds of rivers, or of lands that are frequently flooded, be examined, strata of sand, mud, or gravel will be found, as produced by sediment from the water that has flowed over them; and many contain the remains of the animals that once lived in the water, or were destroyed by its means. Wherever strata, having the same characters, and containing organic remains, are found, it is fair to deduce, how deep soever they may be below the surface, that they are attributable to causes similar to those which are now active in the production of rocks. But such strata have been seen in all those parts of our globe which have been visited by man, and hence it would appear, that the dry land has actually been produced by water; and that the particles which, united together, now form the superficial covering of the earth, must at a former period have floated in loose unconnected particles in pre-existing rivers and oceans.

Taking these statements as the legitimate deductions of geological inquiry, they may be so applied as to account for the present form of the earth. If the earth had been formed

a perfectly spherical body without a motion on its axis, all the water, however distributed on its surface, would have left the places it occupied, and have rushed towards the poles, leaving a ridge of barren mountains round the equator. But if it had been possessed of the same form, and had received an impetus giving it a revolution on its axis, the centrifugal force being greatest at the equator, the water would have been accumulated there, and a mountainous ridge of rocks would have been exposed at the poles.

It may not be perfectly understood by the reader what is meant by the centrifugal force. When a body is made to revolve on its axis, there will always be an attempt in the particles composing that body to fly off from the centre of motion. If a bucket of water be suspended by a string, and a rapid rotary motion be communicated to it, the water will accumulate, and form a sort of wave round the side of the vessel and if we could give the same motion to a substance which consisted of very loosely connected parts, they would fly one from the other, and leave the string by which they were suspended. Now these are the results of the centrifugal or centre-flying force.

But it is well known that the force with which a particle is urged to fly away from the centre of motion increases with its distance from the centre, and consequently, in every globular body, that line of superficial particles intermediate between the points which terminate the axis of revolution, will have the greatest centrifugal force. From this statement it follows, that the equator of the earth must suffer a greater centrifugal force than any other part of its superficies. A very pretty instrument is used by lecturers to illustrate this fact. Two elastic iron hoops are united together, and placed upon an axis, having a capability of compression, so that a pressure upon the top would cause a swelling out of the centre or equator. These hoops are put into motion by a multiplying wheel, or by a band and wheel, which, in consequence of the centrifugal force, causes an expansion of the equator, with an attendant depression of the poles.-(See p. 32.)

Now let it be supposed that the earth, when created, was a perfectly spherical body, and that it had a revolution on its axis, then an immense body of water must have been accumulated round the equatorial regions. But water is everywhere charged with the débris of rocks, which it forms or

collects in its passage from one place to another; and as it has always had the same force under the same circumstances,

it must have done this from the beginning. But rocks are formed by the sediments deposited by water, and the detritus it accumulates; therefore rocks must have been formed in greater amount round the equator than upon any other part of the earth's surface, and hence the greater equatorial di

constantly observed, and necessarily existing in material creation. The limitation of our capacities may render this division or classification necessary, but we must be careful to remember that it is still artificial, and that the sciences are not independent branches of knowledge. This view of the physical sciences will prove the propriety of the effort we are now making, to avail ourselves of their united assistance in explaining the conditions of the earth; and sufficient has already been said to convince the reader that an extensive series of phenomena, such as we behold wherever we may be placed, can only be understood by an acquaintance with many branches of knowledge.

The first doubt as to the perfect sphericity of the earth, is said to have arisen from observations on the pendulum. M. Ritcher, while observing the transits of the fixed stars in the Island of Cayenne, noticed that the pendulum of his clock moved at a rate of 2′ 28′′ a day less than it ought, as regulated by the motion of the sun, and found it necessary to shorten the length of his pendulum nearly one fourth of an inch, in order that it should make vibrations equal to those it made at Paris. This singular phenomenon excited the attention of the astronomer, and, when inquiring into the cause, he was induced to suspect that the earth was not perfectly round.

It may not, however, be quite clear to the reader, what connexion there is between the vibration of a pendulum and the form of the earth, and we may be permitted to illustrate the statement by a few remarks. The instrument we call a pendulum consists of a heavy body suspended by a slight cord or thread, and is frequently used in combinations that are intended for the measurement of time. It must not, however, be supposed, that the pendulum is in any case the moving power; it acts as a regulator in clocks, and the motion originates in the fall of a weight, or in the recoil of a spring attached to the machine. Weights are invariably used in clocks, springs in watches; and the latter are generally regulated by a balance-wheel, and not by a pendulum. The contrivance by which the pendulum of a clock is connected with the train of wheels, and regulates their motion, is called the escapement, and of this there are several varieties, as the lever, and the dead beat, the latter being so named on account of the peculiar sound it produces.