side of the crescent-moon, and her full edge when she is gibbous, are always turned towards the sun. And this explanation, once suggested, would be confirmed, the more it was examined. For instance, if there be near us a spherical stone, on which the sun is shining, and if we place ourselves so that this stone and the moon are seen in the same direction, (the moon appearing just over the top of the stone,) we shall find that the visible part of the stone, which is then illuminated by the sun, is exactly similar in form to the moon, at whatever period of her changes she may be. The stone and the moon being in the same position with respect to us, and both being enlightened by the sun, the bright parts are the same in figure; the only difference is, that the dark part of the moon is usually not visible at all.

Aris

This doctrine is ascribed to Anaximander. totle was aware of it. (Prob. 15.) It could not well escape the Chaldeans and Egyptians, if they speculated at all about the causes of the appearances in the heavens.

Sect. 11.-Eclipses.

THESE Occurrences, from the earliest times, were regarded with a peculiar interest. The notions of superhuman influences and relations, which, as we have seen, were associated, from the earliest times, with the luminaries of the sky, made men look with alarm at any sudden and striking change in those

objects; and as the constant and steady course of the celestial revolutions was contemplated with a feeling of admiration and awe, any marked interruption and deviation in this course, was regarded with surprise and terror. This appears to be the case with all nations at an early period of their civilization.

This impression would cause eclipses to be noted and remembered; and accordingly we find that the records of eclipses are the earliest astronomical information which we possess. When men had discovered some of the laws of succession of other astronomical phenomena, for instance, of the usual appearances of the moon and sun, it might then occur to them that these unusual appearances also might probably be governed by some rule.

The search after this rule was successful at an early period. The Chaldeans were able to predict eclipses of the moon. This they did, probably, by means of their cycle of 223 months, or about 18 years; for at the end of this time, the eclipses of the moon begin to return, at the same intervals and in the same order as at the beginning". Probably this was the first instance of the prediction of peculiar astronomical phenomena. The Chinese have, indeed, a legend, in which it is related that a solar eclipse happened in the reign of Tchong-kang, above 2000

51 The eclipses of the sun are more difficult to calculate; since they depend upon the place of the spectator on the earth.

years before Christ, and that the emperor was so much irritated against two great officers of state, who had neglected to predict this eclipse, that he put them to death. But this cannot be accepted as a real event for during the next ten centuries, we find no single observation, or fact, connected with astronomy, in the Chinese histories; and their astronomy has never advanced beyond a very rude and imperfect condition.

We can only conjecture the mode in which the Chaldeans discovered their period of 18 years; and we may make very different suppositions with regard to the degree of science by which they were led to it. We may suppose, with Delambre", that they carefully recorded the eclipses which happened, and then, by the inspection of their registers, discovered that those of the moon recurred after a certain period. Or we may suppose, with other authors, that they sedulously determined the motions of the moon, and having obtained these with considerable accuracy, sought and found a period which should include cycles of these motions. This latter mode of proceeding would imply a considerable degree of knowledge.

It appears probable rather that such a period was discovered by noticing the recurrence of eclipses, than by studying the moon's motions. After 6585 days, or 223 lunations, the same eclipses nearly will recur.

52 A. A.; p. 212.

It is not contested that the Chaldeans were acquainted with this period, which they called Saros ; or that they calculated eclipses by means of it.

Sect. 12.-Sequel to the Early Stages of Astronomy.

EVERY stage of science has its train of practical applications and systematic inferences, arising both from the demands of convenience and curiosity, and from the pleasure, which, as we have already said, ingenious and active-minded men feel in exercising the process of deduction. The earliest condition of astronomy in which it can be looked upon as a science, exhibits several examples of such applications and inferences, of which we may mention a few.

Prediction of Eclipses.-The cycles which served to keep in order the calendar of the early nations of antiquity, in some instances enabled them also, as has just been stated, to predict eclipses; and this application of knowledge necessarily excited great notice.

Terrestrial Zones.-The globular form of the earth being assented to, the doctrine of the sphere was applied to the earth as well as the heavens; and its surface was divided by various imaginary circles; among the rest, the equator, the tropics, and circles at the same distance from the poles as the tropics are from the equator. One of the curious consequences of this division was the assumption,

that there must be some marked difference in the stripes or zones into which the earth's surface was thus divided. In going to the south, men found countries hotter and hotter, in going to the north, colder and colder; and it was supposed that the space between the tropical circles must be uninhabitable from heat, and that within the polar circles, again, uninhabitable from cold. This fancy was, as we now know, entirely unfounded. But the principle of the globular form of the earth, when dealt with by means of spherical geometry, led to many true and important propositions concerning the lengths of days and nights at different places.

Gnomonick.-Another important result of the doctrine of the sphere was Gnomonick or Dialling. Anaximenes is said by Pliny to have first taught this art in Greece; and both he and Anaximander are reported to have erected the first dial at Lacedemon.

Measure of the Sun's Distance.-The explanation of the phases of the moon led to no result so remarkable as the attempt of Aristarchus of Samos to obtain from this doctrine a measure of the distance of the sun as compared with that of the moon. If the moon was a perfectly smooth sphere, when she was exactly midway between the new and full in position (that is a quadrant from the sun) she would be somewhat more than a half moon; and the place when she was dichotomised, that is, was an exact semicircle, the bright part being bounded by a straight