Having considered the greatest known distances of comets, we will now notice instances of their greatest proximity hitherto measured. This same comet of 1680 approached the sun's surface within one sixth of his diameter. Perihelia which take place beyond the orbit of Mars cannot often be observed from the earth, yet we have no reason to suppose that more lie within than without it. We have an opportunity therefore to observe very few of those comets which actually enter the solar system.

§ 169. The mutual influence of comets and planets has always been a subject of great interest. By these the comet's progress may be retarded or accelerated, the place of its nodes changed, its perihelion distance diminished or increased, and the inclination as well as eccentricity of its orbit altered. And these changes during one revolution are sometimes so considerable as to render the identity of a comet at its successive returns to the sun very doubtful.

Halley's comet first drew the attention of astronomers to these perturbations. After having ascertained its approaches to the sun in the years 1531, 1607, and 1682, Halley was surprised to find that the period of its first revolution was longer by thirteen months than the following one. He thought the difference might be caused by the disturbing action of the planets, particularly Jupiter and Saturn; and after a rough estimate of their attractions he announced the return of the comet for the end of 1758 or the beginning of 1759. The comet appeared as announced, proving the weight of comets and the extent of gravitation to them. During its next revolution this comet will be very much diverted from its course by the planet Uranus.

$170. The comet of 1770 exhibited still more remarkable changes in its orbit. Astronomers had in vain endeavored to represent its observed course by a parabola. At length its orbit was discovered to be an ellipse, not so elongated as to approximate to a parabola, but much shorter, and requiring only a period of five and a half years. This result seemed very extraordinary, since the comet which should to have been so often visible, on account of the shortness of its period and perihelion distance, had never yet been seen on any previous occasion; and the

circumstance was still more unaccountable, when it was found that the comet made no subsequent return to the

sun.

At length, by tracing back the movements of this comet in its orbit, it was found that at the beginning of 1767 it had entered within the sphere of Jupiter's attraction. The amount of this attraction being calculated from the known proximity of the two bodies, the previous orbit of the comet was determined. It must have been an ellipse of greater extent, having a period of fifty years, in which the comet, even when nearest the sun, was still as far distant as Jupiter. It was therefore very evident, why, as long as the comet continued to circulate in this orbit so far from the centre of the system, it never became visible from the earth; and also that the cause of its appearance in 1770 was the disturbing action of Jupiter which constrained it to move in a shorter ellipse and at a less distance from the

sun.

§ 171. Another question of interest is whether comets can act on planets so as to produce perturbations in their course; and also what would be the consequence of a collision between a comet and a planet. The comet of 1770 affords an answer to the first question. From its brilliancy this comet must have been of considerable size, and was even computed to have a diameter nearly thirteen times as large as the moon. On the two occasions above mentioned it is said to have traversed the whole system of Jupiter's satellites, and at each time required four months to free itself from the sphere of his attraction. Yet not the slightest alteration was observed in the motions of these small bodies. The same comet approached so near the earth as to shorten its own revolution by two days, yet what was its reaction on the earth? If its mass had been equal to that of the earth, it would have lengthened our year by two hours, forty-seven minutes. But nice calculations prove that in the length of that year no change exceeding two seconds would have taken place. Hence as 10,027" 2": : mass of earth mass of comet. The comet's mass was less than the 5000 part of the mass of the earth. It is evident therefore that none of the planets are

liable to be carried out of their course by these diminutive bodies.

Other dangers have been apprehended from the approach of comets. It has been feared that the waters of the ocean would be attracted and thus form a deluge. The small mass of comets precludes all danger of this sort. Besides, the ocean would require some time before its inertia would be overcome; and meanwhile the rapid motion of the comet and the rotation of the earth would have presented a different surface of water to the comet.

§ 172. But though proximity is not alarming, it is very different with actual contact. The risk of actual contact is infinitely small when we consider the immense extent of the planetary spaces. Still collision is possible, and its consequences not without interest to us. If the comet and planet were both moving toward the same quarter of the heavens each would glide from the surface of the other, without any very important change in their movements or their physical constitution. But should the directions of their respective courses be directly opposite, the consequences would be far more serious and permanent. The inconsiderable mass of the comet would be compensated by its prodigious momentum, and the planet might be impeded or altogether arrested in its orbit. If the momenta of the two bodies were equal, the progressive motion of both bodies would be destroyed and they would fall into the

sun.

We Te may perhaps see in the heavens such a collision or the consequences of it. The comet of Encke approaches nearer to the planets than any other; it approaches to 360,000 miles distance of Mercury. This circumstance makes a collision between it and Mercury not improbable.

CHAPTER IX.

PHYSICAL ASTRONOMY.

Analogies observable among the Planets. Their general form and their Atmospheres. Internal state of our Globe. Central Heat. Theories accounting for the external appearance of the Earth and Moon. Objections to the theory of Central Heat. Supposed differences of temperature in space. Laplace's Nebular Theory.

§ 173. Let us throw together all we know of the physical state of our system and of the fixed stars, and see if any light is shed on the former state of the universe. We have ascertained that matter exists in the three states in which we find it at the surface of the earth. It is solid in the planets and moons, and undoubtedly also in the nuclei of the sun and stars. It is aeriform at the surface of the sun and stars, as is proved by the polarization of their light, and also in comets, and in the atmospheres of many of the planets, and in ether. It exists in a liquid form in several of the planets, as is proved by the clouds floating in their atmospheres. But in the moon no water is present, and no atmosphere has been detected.

The atmospheres of the planets differ from one another as to color and density. The sun has one or more atmospheres apparently beside the zodiacal light and ether which may in some way belong to him. Of the solid forms of matter we know very little. In Mars the red color of the soil may be distinctly seen; a variety of color is also perceptible on the moon's surface. The shooting stars have the same composition as the earth. On the whole, appearances favor the idea that the composition of the planets is identical or only slightly varied.

§ 174. The general form we have seen is the same in all. All likewise depart slightly from this general form. Their surfaces are irregular. Mountain peaks are discernible in the moon, in the nearer planets, and perhaps in the sun. Gravity accounts for their general form; some upheaving force must have caused the departures from it.

The upheaving force is much weaker than gravity; it only roughens the surface.

We call our globe solid and surrounded by a liquid and a gaseous envelope. It may also have a liquid interior. We know that it increases in density towards the centre. But of the state in which the materials of its crust exist below a small depth we know nothing. In mines and springs the heat of the earth increases about 1° Fahrenheit for 54.5 feet. If we suppose the increase to continue in an arithmetical ratio, a stratum of granite would be in a state of fusion at a depth of twenty-one geographical miles, or at between four and five times the elevation of the highest summit of the Himalaya. Chemical combinations and the neighborhood of volcanoes and the heat imbibed from them, may account for some of the internal heat, but not for so constant an increase and to so great a depth as has been observed in many parts of the solid land. Central heat accounts for it better, and it has therefore been a favorite hypothesis. Central heat also accounts for other appearances on the surface of the earth. Before we adopt it, however, we must see whether no other cause could produce the same appearances on the earth, and we must seek in the other planets, and particularly in our neighbor the moon, for evidence of the existence of central heat there. We must also consider whether from what we know of the formation of the earth the central portions would be likely to remain fluid.

§ 175. Direct researches on our own globe teach us but little. Man's eyes are turned outwards. Accumulating all the facts which the best telescopes reveal, with regard to the distant stars which, strange to say, seem to be undergoing more changes than the humble members of our system, scrutinizing the planets and especially the moon, we may come to some definite conclusion respecting the interior of our globe, and the identity of the materials of the suns and planets. Nay we may form some idea of the circumstances under whose control they took their present shape, and may judge which are the older, which the newer, inhabitants of the heavens.