three miles, sixteen times eight in four miles, always increasing as as the square of the distance.

What the earth loses of its sphericity by moun→ tains and vallies, is very inconsiderable; the highest eminence bearing so little proportion to its bulk, as to be scarcely equivalent to the minutest protube rance on the surface of a lemon.

It is proper, however, to acquaint the young pupil, that though we call our earth a globe, and that, when speaking in general terms, it may be considered as such; yet, in the strictness of truth, it must be obsevred, that it is not exactly and perfectly a sphere, but a spheroid, flattened a little towards the poles, and swelling at the equator; the equatorial diameter being about thirty-four miles longer than the diameter from pole to pole. This difference bears, therefore, too small a proportion to the diameter to be represented on globes. M. Cassini, from Picard's measure of a degree, asserted, that the earth was an oblong or prolate spheroid, flattened at the equator, and protuberant at the poles; while Newton and Huygens, from a consideration of the known laws and the diurnal motion of the earth, concluded that the figure of the earth was that of an oblate spheroid, flattened at the poles, protuberant at the equator. To decide this important question, Louis XIV. ordered two degrees of the meridian to be measured, one under the equator, the other as near the pole as possible. For this purpose, the Royal Academy of Sciences sent Messrs. Vaupertuis, Clairault, Camus, and Le Monnier, to Lapland: they set out from France in 1735, and returned in the spring of the

year 1736, having satisfactorily accomplished the purpose for which they were sent. Messrs. Godin,

Condamine, and Bouguer, were sent on the southern expedition to these the King of Spain joined Don George Juan, and Don Anthony de Uloa, who left Europe in the year 1735; and after, encountering innumerable hardships and difficulties, returned to Europe at different times, and by different ways, in 1744, 1745, 1746. The result of this arduous task was a complete confirmation of Newton's theoretical investigation. The difference between the equatorial and polar dimensions, when compared with the earth's semi-diameter, is but an inconsiderable quantity, amounting in the whole to an elevation of little more than 16 of 3970; that is to less than a 240th part of the distance from the surface of the earth to the centre. If a meridional section of such a spheroid were laid down upon paper, the eye would not distinguish it from a perfect circle.

OF THE DIURNAL MOTION OF THE EARTH.

Though it is this motion which gives us the grateful vicissitudes of day and night, adjusted to the times of labour and rest; yet young people generally find some difficulty in conceiving that, the earth moves ; the more so because, in order to allow it, they must give up, in a great measure, the evidence of their exterior senses, of which the impressions are at their age exceeding strong and lively. It will, therefore, be necessary for the tutor to prove to them that they can, by no means, infer that the earth is at rest because it appears so, and convince them by a

variety of facts, that reason was given to correct the fallacies of the senses.

To this end, we shall here point some instances where apparent motion is produced in a body at rest, by the real motion of the spectator. Let us suppose a man in a ship to be carried along by brisk gale, in a direction parallel to a shore, at no great distance from him; while he keeps his eye on the deck, the mast, the sails, or any thing about the ship, that is to say, while he sees nothing but some part of the vessel on board of which he is, and consequently every part of which moves with him, he will not perceive that the ship moves at all. Let him, after this, look to the shore, and he will see the houses, trees, and hills, run from him in a direction contrary to the motion of the vessel; and supposing him to have received no previous information on the subject, he might naturally conclude; that the apparent motion of these bodies was real.

In a similar situation to this, we may conceive the inhabitants of the earth; who, in early times, knew nothing of the true structure or laws of the universe; saw the sun, the stars, and the planets, rise and set, and perform an apparent revolution about the earth. They had no idea of the motion of the earth, and therefore all this appearance seemed reality. But as it is highly reasonable to suppose, that as soon as the slightest hint should be given to the man, of the motion of the vessel, he would begin to form a new opinion, and conceive it to be more rational, that so small a thing as a ship should move, rather than

all that part of the earth which was open to his view; so, in the same manner, no sooner was an idea formed of the vast extent and greatness of the universe, with respect to this earth, than mankind began to conceive it would be more rational that the earth should move, than the whole fabric of the heavens.

By another familiar instance it will be easy to shew the young pupil, that as the eye does not perceive its own motion, it always judges from appearances. Let a person go into a common windmill, and desire the miller to turn the mill round, while he is setting within, with his eyes fixed on the upright post in the centre thereof; this post, though at rest, will appear to him to turn round with considerable velocity, the real motion of the mill being the cause of the apparent motion of the swivel-post. Seafaring people are furnished with various instances to illustrate this subject; those who are busy in the hold of a ship at anchor, cannot by any perception determine whether the ship has swung round or not by the turn of the tide. When a ship first gets under-way with a light breeze, she may be going at a good rate, before those who are between decks can perceive it. Having thus obviated the objections which arise from the testimony of the senses, we may now proceed to consider the arguments which tend more directly to prove the motion of the earth.

All the celestial motions will, on this supposition, be incomparably more simple and moderate.

This opinion is much more agreeable to our notions of final causes, and our knowledge of the economy of nature; for if the earth be at rest, and the stars, &c. move round it once in 24 hours, their velocity must be immense; and it is certainly more agreeable to reason, that one single body, and that one of the smallest, should revolve on its own axis in 24 hours, than that the whole universe should be carried round it in the same time, with inconceivable velocity.

The rotation of the earth round its axis is analogous to what is observed in the sun, and most of the planets; it being highly probable that the earth, which is itself one of the planets, should have the same motion as they have, for producing the same effect; and it would be as absurd in us to contend for the motion of the whole heavens round us in 24 hours, rather than allow a diurnal motion to our globe, as it would be for the inhabitants of Jupiter to insist that our globe and the whole heavens must revolve round them in 10 hours, that all its parts might successively enjoy the light, rather than grant a diurnal motion to their habitation.

All the phenomena relative to this subject, are as easily solved on the supposition of the earth's motion, as on the contrary hypothesis.

Besides the foregoing considerations, there are several arguments to be deduced from the higher parts of astronomy, which demonstrably prove the diurnal motion of the earth.

Before we enter into a further explanation of phe