the two people must have known how to trace a meridian line, and the Egyptian pyramids are still existing monuments of this step in the ancient science: it must be owned, however, that to orient a building is not a work of much difficulty, when great precision is not required, since it would be only necessary to mark on the ground a line between the spectator and a terrestrial object when the string of a suspended plummet appears to pass through the pole star and that object.

CHAPTER III.

THE NATURE OF THE EARLIEST OBSERVATIONS.

General phenomena of the heavens.-The phenomena of the sun and moon used as measures of time.-The moon's proper motion, and phases, observed. Division of the fixed stars into particular groups. The original constellations changed by the Greeks.-Description of the constellations.— Their designations supposed to relate to the qualities of the seasons.— Astrological notions connected with the constellations.-The sun's proper motion, and the direction of his visible path ascertained. The use of the gnomon in astronomy.-Methods of obtaining a well defined shadow.The length of the year determined by the gnomon.

Now if, in the absence of all positive information concerning the first discoveries relating to the movements of the heavenly bodies, of which indeed scarcely a trace has been preserved, we might be permitted to supply the deficiency under the guidance of such hints as can be collected from the ancient writers, we would venture to assert that they must have taken place nearly in the following manner.

A general and circular movement of the bodies in the firmament would be the first phenomenon recognized even by a superficial observer. The shepherd who watched his flock by night on some plain bounded by the horizon would soon perceive that certain stars rose from the ocean, or from behind the hills which limited his view towards the east, and each, after describing a curve in the heavens, disappeared behind the terrestrial objects about the west; that some performed their revolution without descending so low in any part as to reach the horizon; and, finally, that one star seemed to be stationary in the heavens during all the time that the absence of the sun permitted it to be visible; nor could these phenomena fail to suggest the idea of the revolution of some geometrical figure, as a cone, a cylinder, or a sphere, about a certain line passing through the eye of the observer, and situated obliquely to the plane of the terrestrial horizon. How long any doubt existed concerning the true figure of that surface on which the celestial bodies appear to describe their daily revolutions is uncertain.

We have seen in what manner Aristotle argued in favour of the opinion that the universe is globalar, and such reasoning may, from the first, have been considered as establishing the belief that the starry heaven was of a like form; but the means explained by Proclus, of ascertaining its true figure, have so much the appearance of being those which would present themselves to the minds of the earliest enquirers into the causes of the celestial phenomena, that we cannot do better, in this place, than describe them. This writer first observes that some of the stars evidently describe complete circles daily in the heavens about one fixed point; then, in proof that the movement of the sun is circular, he alleges the inequality in the lengths of the days and nights in different seasons, and the changes gradually made, during the day, in the position of the shadow cast by any terrestrial object; and, lastly, he shews that the sun is, at all seasons, on the surface of one sphere because his apparent magnitude is invariable which, he says, could not be the case if, from midwinter to midsummer, he had moved northwards on the surface of a cylinder or cone; since, in one situation, he would then, evidently, have been nearer to the spectator than in the other, and consequently, he ought to have appeared greater.

However inconclusive this argument may seem to a modern astronomer, whose instruments afford him the means of detecting the periodical variations in the apparent magnitudes of the sun and moon, to the ancients it must have been in the highest degree satisfactory. The planets and fixed stars did not, on account of their smallness, admit of this kind of proof, but the celebrated Euclid, by a comparison of the visible magnitudes of the circles they describe about the pole, demonstrated that the revolving surface to which they belong could be no other than that of a sphere. The results, however, of Euclid's demonstration were, in all probability, very early anticipated, for it appears to have been, from the first, generally understood that the celestial bodies were attached to the concave surface of a spherical shell which revolved in a certain time about the

earth, and that the latter was situated in or near the centre of this concavity.

The phenomena presented by the sun and moon would doubtless next attract the attention of persons impelled to seek information concerning the objects that surround them by that curiosity which, for his improvement, has been so deeply implanted in man. The risings and settings of the former, which afford the means of measuring intervals of time for regulating the ordinary occupations of life, would be immediately attended to, and employed for that purpose; the inequalities of the periods of alternate light and darkness being for some time, perhaps, disregarded.

The remarkable variations in the apparent form of the moon, by recurring constantly in a certain order would, no doubt, be the indices by which the first steps were taken for the division of time into periods exceeding those marked out by the intervals between the sun's rising and setting; and, by the inhabitants of a maritime region, it is probable that those changes would be suspected to have some connexion with the changes in the elevations of the waters on the coast or in the mouths of rivers. It is easy to observe that a portion of time equal to that including about thirty repetitions of the sun's risings would be comprehended between two successive appearances of the moon in the crescent form; hence, such a phenomenon would be a convenient signal by which to regulate the celebration of some periodical festival, or the commencement of a journey to some distant place; indeed, we find, among all nations not enlightened by science, that the first appearance of the new moon, as it is called, is made use of for those purposes, and one of the duties of an established priesthood seems, at first, to have been the looking out for, and giving notice of that event. The times at which the moon presents a full round orb being found to occur about the middle of the interval of two of her successive appearances in the crescent form would, also, be made available for the same purposes, and even the various phenomena of the increase and wane might be objects of attention, but the times of their accomplishment being marked less precisely would not, perhaps, be so generally useful. The appearances of particular

stars with the sun, either on the same or on opposite sides of the horizon would also, as we have shewn, afford indications of times useful for many of the purposes of human life; but it is probable that these were not attended to till long after the more remarkable phenomena of the sun and moon had been made subservient to those to which they are applicable.

The contemplation of the moon's progress through the heavens would, in a few nights, lead to the discovery of a twofold movement in that luminary; for it could not escape observation that while, like the rest of the heavenly bodies, she moved during the night from east to west, she gradually receded from certain stars and approached others situated towards the east of them by a movement contrary to the former: and, if her position with respect to some remarkable star seen after sunset on any night, was compared with her position with respect to the same star on the following night, it would be found that she had made a movement towards the east, during the intervening time, of more than one thirtieth part of the whole circumference of the celestial sphere. The phenomena consequent upon this movement take place in the following manner, and they must have been so observed from the first moment the heavens were regarded with the least attention.

The moon first becomes visible near the setting sun and then she has the form of a slender crescent of light. At the close of the following day she appears to have receded considerably from the sun towards the east, she sets later and her luminous face is found to have increased in breadth. This deviation from the sun and the augmentation of breadth continue to increase daily and, in less than seven days, she is, at the setting of the sun, distant from that luminary as much as one quarter of the circumference of a circle surrounding the heavens, and her form is that of a semicircle. At the end of about seven days and a half from this time she has receded from the sun to the extent of half the circumference of such a circle; she appears to rise in the east when the sun is setting and her brilliant disc, which had assumed an oval form by gradually increasing in breadth, becomes an entire circle, in which state she is designated the