any farther particulars relating to eclipses, and especially into any details of a practical character. The reader will not, however, it is hoped, be satisfied with this outline, as he may easily acquaint himself with the method by which these phenomena are predicted; and will find more satisfaction in determining the time when an appearance of this kind will be seen, and will acquire more information by doing so, than he could obtain from such details as are suited to the character and objects of this volume.

This phenomenon is one of great importance to us, and must also be so to the inhabitants of the moon, if that planet be inhabited by beings of similar capabilities and wants as ourselves. By a lunar eclipse, the opinion that the earth is a spherical body is confirmed, for the shadow of the earth upon the lunar disk is always bounded by an arc of a nearly circular curve. Now this could not be the case if the shadow were not conical, and the shadow could not be conical if the earth were not spherical. It may also be deduced from the same phenomenon, that the sun is larger than the earth, and that the earth is larger than the moon; for if the sun were not larger than the earth, the shadow could not converge, or end in a point; and if the earth were not larger than the moon, the latter could never be totally eclipsed; but the earth's shadow envelops it at the distance of the lunar orbit. By eclipses, and especially those of the moon, the longitude of places may be determined, the latter being peculiarly adapted for the solution of this problem, because the appearance is the same in all places where it is visible.

It must not, however, be forgotten, that the calculation of the periods when eclipses have occurred has sometimes assisted in determining the time of historical events; and thus astronomy has lent its aid to literature. It is stated by Thucydides, that a solar eclipse was observed at Athens in the afternoon of a summer's day, in the first year of the Peloponnesian war; and it was so nearly total that the stars made their appearance. By calculation, it is found that this happened at about six o'clock in the evening, on the third of August, in the year 431 before Christ. When it is remembered that all our divisions of time are founded upon the motions of the heavenly bodies, and the phenomena which are thus produced, it will not appear singular that the record of eclipses should often assist the chronologist in the determi

nation of dates. The paths, periods, and irregularities of the moon and the earth being known, it is not a very difficult task to determine the times when eclipses have occurred. An industrious and persevering man might easily compute the times when the sun or the moon has been eclipsed, as well as the time when they will be; and he may thus frequently ascertain the dates of remarkable, political, or national events. The attention of men has ever been directed to a consideration of the appearances exhibited in the heavens, and they seem to have been always conscious that no terrestrial phenomenon could be employed to measure the lapse of time. These → two circumstances may probably account for the frequent allusion to astronomical occurrences in the works of the ancient historians and poets.

There are other celestial appearances, besides those which have been described, that are occasionally seen from the earth; but those that have been mentioned are the most remarkable. It is seldom that men take an enlarged view of the influence of scientific knowledge upon the minds of individuals. They speak of it as calculated to raise the intellectual standard, to free the individual from the dominant control of superstition or fear, and to give him great capacities of generalization. But the advantages of knowledge can be appreciated only when we can perfectly realize the condition of a society in which it had never been acquired, and upon which it had exerted no influence by the medium of others; if, indeed, a society could exist under such conditions. Compare the feelings which an individual of that community would experience during a solar eclipse with those which would be indulged by one who, though he could not account for it, had made himself acquainted with laws which govern some of the most remarkable celestial phenomena. The one would suffer an uncontrolled fear by an appearance that he must have heard of by tradition, or perhaps may have himself beheld; the other, under full conviction of the propriety of the causes with which he is acquainted, would be induced to seek some information concerning the origin of this newly-exhibited effect, and would be ashamed to entertain, and much more to express, a fear, until he knew it to be justified by the cause. Our object, however, in describing the celestial relations of the earth, and the astronomical phenomena seen upon the surface, has been to give the reader as

comprehensive a view as possible of the position of the earth in the universe, and the means by which it is retained in its position, feeling certain that the personal advantages, to which we have occasionally alluded, will be enjoyed by every successful inquirer.

CHAPTER III.

THE ATMOSPHERE AND ITS PHENOMENA.

THE earth is surrounded by an immense aerial ocean, which is an important agent in supporting animal and vegetable life, and in sustaining the present condition of the phenomena around us. An atmosphere is not the necessary appendage of a world; but we have evidence, both within and without us, to prove that the earth is enveloped by a gaseous medium, and that it is of no small importance in the economy of terrestrial conditions. Were we to select from the phenomena we occasionally behold a few that might prove the existence of an atmosphere round our planet, we might mention the resistance it offers to bodies in motion, the force with which at other times it propels them, and the colour it gives to the ethereal vault.

PROOFS OF THE EXISTENCE OF ATMOSPHERE.

Every body in rapid motion is retarded by the atmosphere through which it moves. A ball, or any other substance, when once put into motion, would continue in that state for ever if there were no disturbing force. Matter has no predisposition to rest, and the ancient philosophers described its habitude very incorrectly, when they compared it to an idle man capable of motion, but much attached to rest. Motion and rest are relative conditions; and when a body is in one or the other, it is because of the forces that are acting upon it. Matter is in fact perfectly passive, and therefore matter once in motion would continue in motion for ever if there were no force tending to bring it to a state of rest. The resistance of the air is one of these. The pendulum, for instance, when put into motion, vibrates in obedience to the force of gravity,

and would never come to rest if its motion were not destroyed by friction and the resistance of the air.

The atmosphere itself is capable of motion; and when its equilibrium is disturbed, either locally or generally, it produces by impact an effect upon all bodies that are in its path. It not only carries away in its progress the lighter substances with which it comes into contact, but, when greatly agitated, uproots trees, crumbles rocks, and overturns buildings. Man, who subdues and regulates all natural agents by the exercise of those noble properties of mind with which God has blessed him, has applied air in motion as a mechanical force, and compels it to accomplish his wishes, not only in the allevia-` tion of his daily toil, but in facilitating the intercourse between the several sections of the human family; and as though these adaptations were not sufficient to prove the superiority of mind over matter, it is often made to minister to his pleasures, both of mind and of appetite.

If it were necessary to mention any other phenomenon as proving the existence of an atmosphere, allusion might be made to its colour. The vault of heaven, when uncovered by the clouds which sometimes hang as draperies beneath it, has a beautiful azure or blue tinge. This colour cannot of course belong to space, nor is it the result of the influence of those bodies which revolve in it, but is occasioned by the passage of light through the atmosphere. When a small quantity is examined, the colour cannot be detected, because the portion of coloured light transmitted to the eye is too faint to give the sensation of colour; and, for the same reason, a bottle of sea water has a clear transparent appearance, though the deep sea from which it was taken may have a rich green colour. By such facts as these we are made acquainted with the existence of an atmosphere surrounding the earth, and extending to a considerable height above its surface.

The atmosphere is highly important as being the cause of many phenomena we behold, in modifying the influence of others, and in its essential character as the supporter of animal and vegetable life. It has been ascertained by chymists that no other combination of the gases with which we are acquainted would serve the same purpose as that which has been employed, but would be either instantaneously or progressively destructive of life. The atmosphere is also the conductor of sound, gives buoyancy to the clouds, and capa

bilities of flight to winged animals. If man were able to exist without an atmosphere, he would exist without a knowledge of pleasure, and the expenditure of muscular strength would be so much increased as greatly to augment the burden of the curse under which he labours.

COMPOSITION OF AIR.

The body of gaseous matter by which the earth is surrounded is composed of two elastic fluids, called oxygen and nitrogen, in the proportions of one part of the former to four of the latter. But the atmosphere contains other substances, which must be rather considered as impurities than as absolutely necessary for its composition, and these are consequently in variable proportions. Carbonic acid gas is a principle commonly found in atmospheric air; Saussure found it in that which he brought from the top of Mont Blanc; and Humboldt, in that from near the summit of the Andes. It is however now certain that the proportion of carbonic acid in air not only varies in different places, but also at different seasons of the year; and it is possible that it may be sometimes absent, as the companions of La Perouse failed to detect its presence on the top of the peak of Teneriffe.

A most interesting series of experiments has been made by M. Saussure the younger, at Chambeisy, near Geneva, from which it appears that meteorological changes and seasons have a tendency to alter the proportions of carbonic acid in the air. A long-continued frost increases, and a thaw decreases, the proportion; heavy rains were also found to diminish its quantity, probably by dissolving it; and on the same principle we may account for the circumstance, that there was less of this gas in the air over the Lake of Geneva than at Chambeisy. The philosopher ascertained that there is more in the three winter than in the three summer months; and at night than in the day; and he estimates the average proportion of carbonic acid at 4.15 volumes in 10,000 of atmospheric air.

Aqueous vapour is also present in the atmosphere in variable quantities between one and one and a half per cent. ; and many gaseous bodies may be detected; for all the substances that can become aerial fluids at common temperatures must be occasionally found. But these are locally distributed, as well as variable in amount.