LECTURE XXXVIII.

EXPLANATION OF THE SEASONS, AND OTHER PHENOMENA, ON THE COPERNICAN SYSTEM.

AM now going to consider the earth as a planet; having already given you an outline of the folar fyftem, of which the fun is the center, with the feven planetary globes revolving in their refpective orbits around him. The earth we inhabit is one of these seven revolving planets, and completes it's revolution in 365 days, 5 hours, 49 minutes, which conftitutes our year; for it is by this progreffion or annual motion of the earth, that our year is meafured. But befides this, in the space of 24 hours it makes one complete revolution on it's axis, by which motion day and night are alternately occafioned all over the world. To explain the phenomena on these principles, and to remove objections and difficulties, will be the fubject of this Lecture; and, first of all, it will be neceffary to prove to you the globular form of our earth.

OF THE SHAPE OR FIGURE OF THE EARTH.

I have already obferved, that the appearance of the heavenly bodies is not the fame to the inhabitants of various parts of the earth; that the fun, the moon, and the ftars, rife and fet in Greenland in a manner very different from what they do in the East Indies, and in both places very different from what they do in England: and as it was naVOL. IV. tural

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tural to attribute the caufe of this change in the apparent face of the heavens, to the figure of the earth, (for appearances must ever anfwer to the form and ftructure of the things,) the nature of this figure was, therefore, one of the first objects of inquiry among philofophers and aftronomers.

Some fages of antiquity concluded, that the earth muft neceffarily be of a fpherical figure, becaufe that figure was, on many accounts, the most convenient for the earth, as an habitable world: they alfo argued, that this figure was the moft natural, becaufe any body expofed to forces, which tend to one common center, as is the cafe with the earth, would neceffarily affume a round figure. The affent, however, of the modern aftronomers to this truth, was not determined by fpeculative reafoning; but on evidence, derived from facts and actual obfervation. From thefe I fhall felect those arguments, that I think will have the greatest weight.

It is known, from the laws of optics and per1pective, that if any body, in all fituations, and under all circumftances, projects a circular fhadow, that body must be a globe.

It is alfo known, that eclipfes of the moon are caufed by the fhadow of the earth.

And we find, that whether the fhadow be projected towards the caft or the weft, the north or the fouth, under every circumftance it is circular; the body, therefore, that cafts the fhadow, which is the earth, must be of a globular figure.

You will obtain another convincing proof of the globular shape of the earth, by inquiring in what manner a person standing upon the coast of the fea, and waiting for a veffel which he knows is to arrive, fees that veffel. We fhall find, that he firth of all, and at the greateft diftance, fees the top of the maft rifing out of the water; and the ap

pearance

pearance is, as if the fhip was swallowed up in the water. As he continues to obferve the object, more and more of the maft appears; at length he begins to fee the top of the deck, and by degrees the whole body of the veffel. On the other hand, if the fhip be departing from us, we firft lofe fight of the hull, at a greater diftance the main-fails difappear, and at a ftill greater the top-fail. But if the furface of the fea were a plane, the body of the ship, being the largest part of it, would be seen first, and from the greatest distance, and the mafts would not be vifible till it came nearer.

To render this, if poffible, ftill clearer, let us confider two fhips meeting at fea, the top-maft of each are the parts firft difcovered by both, the hull, &c. being concealed by the convexity of the globe which rifes between them. The fhips may, in this inftance, be refembled to two men, who approach each other on the oppofite fides of a hill; their heads will be firft feen, and gradually, as they approach, the body will come entirely in view. From hence is derived a rational method of eftimating the distance of a ship, which is in use among fea-faring people, namely, of obferving, HOW LOW THEY CAN BRING HER DOWN; that is to fay, the man at the maft-head fixes his eyes on the veffel in fight, and flowly defcends by the fhrowds, till fhe becomes no longer vifible. The lefs the distance, the lower he may defcend before the difappears. If obfervations of this kind be made with a telescope, the effect is fill more remarkable; as the diftance incrcafes or diminishes, the fhip in fight will appear to become more and more immerfed, or to rife gradually out of the

water.

This truth is alfo fully evinced by the following confideration; that hips have failed round the earth, have gone out to the weftward, and have E 2

come

come home from the eastward; or in other words, the fhips have kept the fame course, and yet returned from the oppofite fide into the harbour whence they firft failed. Now we are certain that this could not be the cafe, if the earth were a plane; for then a perfon, who fhould fet out for any one point, and go on ftrait forward, without ftopping, would be continually going further from the point from which he fet out.

Fig. 1 and 2, pl, 2, are illuftrations of the foregoing principles. Fig. 1, fhews that if the earth was a plane, the whole of a ship would be seen at once, however diftant from the spectator, and that, whether he be placed at the top or bottom of a hill. From fig. 2, it appears, that the rotundity of the earth, represented by the circle A B C, conceals the lower part of the ship d, while the top-maft is ftill visible; and that it is not till the fhip comes to e that the whole of it is vifible.

The following remarks evince the fame truth. Obferve any star near the northern part of the horizon, and if you travel to the fouth, it will feem to dip farther and farther downwards, till by proceeding on, it will defcend entirely out of fight. In the mean time, the flars to the fouthward of our traveller will feem to rife higher and higher. The contrary appearances would happen, if he went to the northward. This proves that the earth is not a plane furface, but a curve in the direction fouth and north. By an obfervation nearly fimilar to this, the traveller may prove the curvature of the earth, in an eaft and weft direction.

The globular figure of the earth may be alfo inferred from the operation of levelling, or the art of conveying water from one place to another: for in this procefs, it is found neceffary to make an allowance between the true and apparent level; or in other words, for the figure of the earth. For

the

the true level is not a frait line, but a curve which falls below the strait line about eight inches in a mile, four times eight in two miles, nine times. eight in three miles, fixteen times eight in four miles, always increasing as the fquare of the dif

tance.

What the earth lofes of it's fphericity by mountains and vallies, is very inconfiderable; the highest eminence bearing fo little proportion to it's bulk, as to be fcarcely equivalent to the minuteft protuberance on the furface of a lemon.

It is proper, however, to acquaint you, that though we call our earth a globe, and that when fpeaking in general terms, it may be confidered as fuch; yet in the ftrictnefs of truth, it must be obferved, that it is not exactly and perfectly a fphere, but is a pheroid, flattened a little towards the poles, and fwelling at the equator; the equatorial diameter being about thirty-four miles longer than the diameter from pole to pole.

OF THE DIURNAL MOTION OF THE EARTH.

Though it is this motion which gives us the grateful viciffitude of day and night, adjusted to the times of labour and reft; yet most people find fome difficulty in conceiving that the earth moves; the more fo, becaufe, in order to allow it, they muft give up, in a great measure, the evidence of their exterior fenfes, of which the impreffions are exceeding strong and lively. It will, therefore, be neceffary to prove to you, that you can by no means infer that the earth is at reft, because it appears to be fo, and to convince you by a variety of facts, that reafon was given to correct the fallacies of the fenfes.

To this end we shall here point out fome inftances,

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