diameter from pole to pole. This difference bears, however, too fmall a proportion to the diameter, to be represented on globes.

OF THE DIURNAL MOTION OF THE EARTH.

Young people generally find some difficulty in conceiving that the earth moves; the more fo, because, in order to allow it, they must give up, in a great measure, the evidence of their exterior fenfes, of which the impreffions are with them. exceeding strong and lively. It will, therefore, be neceffary for the tutor to prove to them, that they can by no means infer that the earth is at rest, because it appears fo to them; and he fhould convince them, by a variety of facts, that reafon was given to correct the fallacies of the senses. To this end we shall here point out fome inftances, where apparent motion is produced in a body at rest, by the real motion of the Spectator.

Let us fuppofe a man in a fhip, to be carried along by a brisk gale, in a direction parallel to a shore, at no great distance from him; while he keeps his eye upon the deck, the maft, the fails, or any thing about him in the fhip; that is to say, while he sees nothing but fome part of the veffel

on board of which he is, and confequently every part of which moves with him, he will not perceive that the fhip moves at all. Let him, after this, look to the shore, and he will fee the houses, trees, and hills, run from him in a direction contrary to the motion of the veffel; and fuppofing him to have received no previous information on this fubject, he might naturally conclude, that the apparent motion of these bodies was real.

In a fimilar fituation to this, we may conceive the inhabitants of the earth; who, in early times, knowing nothing of the true ftructure or laws of the univerfe, faw the fun, the ftars, and the planets, rife and fet, and perform an apparent revolution about the carth. They had no idea of the motion of the earth, and therefore all this appearance feemed reality. But as it is highly reasonable to suppose, that as foon as the slightest hint fhould be given to the man, of the motion of the veffel, he would begin to form a new opinion, and conceive it to be more rational, that fo fmall a thing as the fhip should move, rather than all that part of the earth which was open to his view; so, in the fame manner, no fooner was an idea formed, of the vast extent and greatness of the univerfe, with respect to this earth, than mankind began to conceive it would

be

be more rational that the earth should move, than the whole fabric of the heavens.

By another familiar instance, it will be easy to fhew the young pupil, that as the eye does not perceive it's own motion, it always judges from appearances. Let a perfon go into a common windmill, and defire the miller to turn the mill round while he is fitting within it, and his eyes fixed on the upright poft in the center thereof; this poft, though at reft, will appear to him to turn round with confiderable velocity, the real motion of the mill being the cause of the apparent motion of the fwivel poft. Having thus obviated the objections which arife from the testimony of the fenfes, we may now proceed to confider the arguments which tend more directly to prove the motion of the earth.

All the celestial motions will, on this fuppofition, be incomparably more fimple and moderate.

This opinion is much more agreeable to our notions of final caufes, and our knowledge of the œconomy of nature; for if the earth be at reft, 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 fingle body, and that one of the fmalleft, fhould revolve on it's own axis in 24 hours, than that the whole universe should be carried round it, in the fame time, with inconceivable velocity.

The rotation of the earth round it's axis is analogous to what is obferved in the fun, and most of the planets; it being highly probable, that the earth, which is itself one of the planets, fhould have the fame motion as they have, for producing the fame effect: and it would be as abfurd 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 infift that our globe, and the whole heavens, muft revolve round them in ten hours, that all it's parts might fucceffively enjoy the light, rather than grant a diurnal motion to their habitation.

All the phænomena relative to this fubject, are as cafily folved on the fuppofition of the earth's motion, as on the contrary hypothefis. The truth of this pofition is pleasingly illuftrated by the armillary sphere, which is fhewn, fig. 2, pl. XIII. The exterior circles reprefent the sphere of the heavens; within these, and in the center of the

fphere,

fphere, is placed a little globe, supported by a steel axis. a and b are two milled nuts. By moving the nut a, the small globe may be turned round the fame way we suppose our earth to revolve, while the outer part, or sphere, remains fixed; but if the nut b be turned the contrary way, the sphere will move round the globe, the fame way as the heavens appear to move.

Thus by this machine, the real motion of the earth round it's axis, within the sphere of the heavens, or the apparent motion of the heavens round the earth, may be represented: and it will fhew, that the result of the various problems are the fame, whether we suppose the heavens to move round the earth, or the earth to revolve on it's axis.

Befides the foregoing confiderations, there are several arguments to be deduced from the higher parts of aftronomy, which demonftrably prove the diurnal motion of the earth.

DEFINITIONS.

Before we enter into a further explanation of phænomena, it will be neceffary to define fome of the principal circles of the globe. The reader