when in that beams of the sun, which are so bent into the earth's shadow, in their passage through the atmosphere, as to afford us a sufficient quantity of light to render the moon visible. There have, however, been eclipses of the moon, part of her orbit nearest the earth, in which she has entirely disappeared; but these instances are very rare. Hevelius mentions one of this kind, which happened on the 25th of April, 1642, when he was not able to distinguish the place of the moon, even with a good telescope, although the sky was sufficiently clear for him to see stars of the fifth magnitude. From what has been said, it is also plain, that there may be a total eclipse of the moon, although she be not exactly in either of the nodes at the time the eclipse happens; for as the diameter of the earth's shadow is much greater than that of the moon, it is plain that she may be wholly involved in the dark cone without passing directly through its axis. The moon may likewise be at such a distance from the node, that only a part of her body can enter the earth's shadow, and then we shall have a partial eclipse of the moon, which will be greater or less according to her situation. But when it happens that the full moon takes place exactly in one of the nodes, then the axis of the earth's shadow will pass through the centre of the moon, and it will be a total and central eclipse. The diameters of the sun and moon are supposed to be divided into twelve equal parts, (Pl. xvIII. fig. 3.) and an eclipse is said to be of so many digits, according to the number of those parts which are involved in darkness. It must also be observed, that an eclipse of the moon always begins on her eastern side, and goes off on her western; and that an eclipse of the sun begins on his western side, and goes off on his eastern. And all that the moon is eclipsed above twelve digits, shows how far the shadow of the earth extends over her body, on that edge to which she is nearest at the middle of the eclipse. Eclipses of the sun are more frequent that those of the moon, because his ecliptic limits are greater; and yet we have more visible eclipses of the moon than of the sun; which is owing to their being seen from all parts of the earth, where the moon is above the horizon at the time the eclipse happens; whilst those of the sun can only be observed on that small portion of the hemisphere on which the moon's shadow falls. The greatest number of eclipses, of both luminaries, which can happen in a year, is seven, and the least two; but the most usual number is four; and it is very rare that there are more than six, one half of which are generally invisible at any particular place. These are the principal particulars relating to the doctrine of eclipses, which admit of a familiar illustration, and if they be properly considered, it will not be difficult to conceive how astronomers are able to foretel the exact time when any phænomenon of this kind will happen. For as an eclipse can only take place at the time of a new or full moon, the chief requisites are to determine the number of mean conjunctions and oppositions that will arrive in every year, and the true places of the sun and moon in their orbits at each of those times. And if from this it appears, that the two luminaries are within the proper limits of the node, there will be an eclipse, or otherwise not, agreeably to what has been already observed upon this subject. But in order to facilitate these operations, we have astronomical tables ready computed, from the theory of gravitation, by which the places of the heavenly bodies, and every other necessary particular, may be easily found for any given instant of time. Dr. Halley has also given a catalogue of all the eclipses that took place from the year 1701 to 1718, which the author of "L'Art de verifier les Dates," and others, continued up to the year 1800; so that by means of the period of nineteen years, in which there is found to be nearly a regular return of the same eclipses, it is easy to institute a calculation, that will determine, to a great degree of precision, the times in which they will happen. This method, however, is not strictly scientific ; and there are, besides, many other elements employed in calculating the quantity and duration of eclipses, which, as you are unacquainted with the higher parts of mathematics, it is scarcely possible to explain in a clear and satisfactory manner. If what has been said should lead you to wish for a farther acquaintance with this doctrine, you will find it amply treated of by Lalande, in his Astronomy, Vince, and others; but these authors cannot be read to any advantage, till you have obtained a previous knowledge of many other branches of science. LETTER XXIV. OF THE NEW PLANETS, AND OTHER DISCOVERIES. Ir is a general and immemorial tradition, which is countenanced both by sacred and profane history, that prodigious changes and revolutions have taken place in our globe since its first formation : and the bare inspection of the earth gives great weight to this opinion. We can perceive, in many instances, that the waters of the ocean have not always been confined within their present bounds. The vegetables and fishes of India, which are found in the petrifactions of Europe; and the number of shells, and other marine productions, discovered in ranges of mountains very remote from the sea, can be accounted for upon no other principle. This was a doctrine which was taught both by Pythagoras and his followers; and Ovid, in explaining the tenets of that sect, speaks in the name of all the Oriental philosophers, when he says, "The face of places, and their forms, decay; |