Imágenes de página

and declination 11° 4^' W. similar to a star of the 5th magnitude, and iiot ascertained to be at a distance different from the three others.

The places of the Georgium planet, only, are annually published in the Nautical Ephemeris, and that of White's; and it can only be from a continual series of accurate observations for a length of time, that proper data can be obtained to construct true elements from.


Having given a general idea of the Copernican system, and the bodies of which it is composed, it will be necessary to enlarge these ideas by a more minute description of the particular parts which form this great whole; and to strengthen them by the force of that evidence, on which the system is founded.


The places of the heavenly bodies could not be settled with accuracy from observations made on the surface of the earth, unless its figure and magnitude were previously known; and, without this knowkdge, computations from the observations of the heavenly bodies, for ascertaining the situation of places on the earth, could not be depended on.

I have already observed, that the appearance of the heavenly bodies is not the same to the inhabitants of various parts of the earth; that the sun, the moon, and the stars, rise and set in Greenland in a manner very different from what they do in the East Indies, and in both places very different to what they do in England: and, as it was natural to attribute the cause of this change in the apparent face of the heavens to the figure of the earth (for appearances must ever answer to the form and structure of the things), the nature of this figure was, therefore, one of the first objects of inquiry among philosophers and astronomers.

Some of the sages of antiquity concluded, that the earth must necessarily be of a spherical figure, because that figure was, on many accounts, the most convenient for the earth', as an habitable world: they also argued, that this figure was the most natural; because any body exposed to forces, which tend to one common centre, as is the case with the earth, would necessarily assume a round figure. The assent, however, of the modern philosopher to this truth, was not determined by speculative reasoning, but on evidence derived from facts and actual observation. From these I shall select those arguments, that I think will have the greatest weight with young minds.

It is known, from the laws of optics and perspective, that if any body, in all situations, and under all circumstances, project a circular shadow, that body must be a globe.

It is also known, that eclipses of the moon are caused by the shadow of the earth.

And we find that, whether the shadow be projected towards the east or west, the north or the south, under every circumstance it is circular: the body, therefore, that casts' the shadow, which is the earth, must be of a globular figure.

We shall obtain another convincing proof of the globular shape of the earth, by inquiring in what manner a person standing upon the coast of the sea, and waiting for a vessel, which he knows is to arrive, Sees that vessel. We shall find that he, first of all, and at the greatest distance, sees the top of the mast rising out of the water; and the appearance is as if the ship was swallowed up in the water. As he continues to observe the object, more and more of t;he mast appears; at length he begins to see the top of the deck, and by degrees the whole body of the vessel. Qn the other hand, if the ship be departing from us, we. first lose sight of the hull, at a greater distance; the main-sails disappear, at a still greater the top-sails. But if the surface of the sea 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 masts would not be visible till it came nearer.

To render this, if possible, still clearer, let us consider two ships meeting at sea, the top-mast of each are the parts first discovered by both, the hull, &c. being concealed by the convexity of the globe which rises between them. The ships may, in this instance, be resembled to two men, who approach each other on the opposite sides of a hill; their heads will be first seen, and gradually, as they approach, the body will become entirely in view. From hence is derived a rational method of estimating the distance of a ship, which is in use among sea-faring people, namely, of observing how low they can bring her down; that is to say, the man at the mast-head fixes his eyes on the vessel in sight, and slowly descends by the shrouds, till she becomes no longer visible. The less the distance, the lower he may descend before she disappears. If observations of this kind be made with a telescope, the effect is still more re-, markable; as the distance increases or diminishes, the ship in sight will appear more and more immersed, or to rise gradually out of the water.

This truth is fully evinced by the following consideration; that ships have sailed round the earth, have gone out from the westward, and have come-, home from the eastward; or, in other words, the ships have kept the same course, and yet returned from the opposite side into the harbour whence they first sailed. Now we are certain that this could not be the case, if the earth were a plane; for then a person, who should set out for any one point, and go pn straight forward without stopping, would be continually going further from the point from which he get out. This argument may be much elucidated, by referring the pupil to a modern terrestrial globe, on which he may follow the tracks of an Anson and a Cook round the world. .

Plate 2, Jig, 1 and 2, are illustrations of the foregoing principles. Fig-l) shews that, if the earth was a plane, the whole of a ship would be seen at once, however distant from the spectator, and that whether he was placed at the top or bottom of a hill. From Jig. 2, it appears that the rotundity of the earth, represented by the circle ABC, conceals the lower part of the ship d, while the top-mast is still visible; and that it is not till the ship comes to e, that the whole of it is visible.

The following remarks evince the same truth. Observe any star nearer the northern part of the horizon, and if vou travel to the south, it will seem to dip farther and farther downwards, till, by proceeding, it will descend entirely out of sight. In the meantime, the stars to the southward of our traveller will seem to rise higher and higher. The contrary appearances would happen, if he went to the northward. This proves that the earth is not a plane surface, but a curve in the direction south and north. By an observation nearly similar to this, the traveller may prove the curvature of the earth, in an east and west direction.

The globular figure of the earth may be also inferred from the operation of levelling, or the art of conveying water from one place to another; for, in this process, it is found necessary to make an allowance between the true and apparent level ; or, in other words, for the figure of the earth. For the true level is not a straight line, but a curve which falls below the straight line about eight inches in a mile, four times eight in two miles, nine times eight in

« AnteriorContinuar »