analogy, but from a series of telescopic observations continued for ten years on the surface of Venus, by the celebrated Schröeter, who has estimated the height of its principal mountains as varying from 10.84 to 22:05 miles. So that after making due allowance for the uncertainty of such observations, it is probable that he has arrived at an approximation to the truth. For, as Venus is 1·40 times nearer to the sun than the earth, and as the heating power of the sun is inversely as the squares of the distance, Venus must receive more caloric than a given area of the earth, in the ratio of 1.96, or nearly as 2 to 1, because the square of 1:40 is 1.96. For the same reason, as the heating power of the sun is 6.959 times greater at Mercury than on the earth, which is 2.638 times farther from the sun, the mountains of Mercury ought to be of corresponding magnitude. And as Mars is 1-494 times farther from the sun than the earth, he must receive 2.232 times less caloric; which must proportionally modify all the conditions of his surface. In like manner, as Jupiter is 5.157 times farther from the sun than the earth, he must receive 26.594 times less caloric. And so on to the extremity of the solar system.

But in estimating the changes which the surface of our planet has undergone during long geological epochs, we must not overlook the influence of variations in the inclination of the

earth's axis, in modifying the mean temperature of different latitudes. It is generally supposed to have been demonstrated theoretically by Laplace, that the total variation of the planetary inclinations must be comprised within the narrow limits of 3°. That in so vast and complicated a problem as that of perturbation, geometers may have overlooked some important elements in their calculations, would seem highly probable from the limited period since which accurate observations have been made; for they admit that the disturbing influence of all the planets and their satellites upon each other is such, that millions of years are required to bring about one cycle.

But if it were wholly impossible to determine the exact amount of variation in the planetary inclinations, during the countless ages that have past, geology affords the most conclusive evidence, that at some remote period of the earth's existence, the inclination of its axis must have been far less than at present; or the equator and poles must have been reversed. For it has been discovered that the secondary formations at Melville Island, and other parts of the polar regions, are filled with the fossil remains of plants and animals, which could have been produced only in a warm climate; and that the newest tertiary deposits in Siberia, abound with bones of the Mastodon or mammoth, which seems to have been as common in the higher

latitudes of Asia and America, as elephants are now within the tropics. The conclusion is therefore undeniable, that a warm climate once prevailed in the arctic regions, and, perhaps, at several distant periods of time. Nor is it possible to account for such a state of things without supposing that in the progress of long geological epochs, the inclination of the earth's axis has undergone very great changes.

That this inclination is now actually diminishing at the rate of one minute in 1261 years, is admitted by all astronomers. It therefore follows, that if it should go on decreasing in the same ratio, without any oscillations or retrogradations, it would wholly vanish in about 177,760 years, when the sun would shine upon every part of the earth with nearly the same force, and produce a grand physical millenium,* or what may be termed

In the tenth book of the sublimest song in our language, Milton represents the earth as enjoying this happy exemption from winter, before the fall of man: after which, the Creator

"Bid his angels turn askance The poles of earth twice ten degrees and more:

Of seasons to each clime; else had the spring

Perpetual smiled on earth with verdant flowers.
These changes in the heavens, though slow, produced
Like change on sea and land.

Beast now with beast 'gan war, and fowl with fowl,
And fish with fish; to graze the herb all leaving,
Devoured each other."

a vernal period of the great year. For although there would be perpetual summer at the equator, and a gradual diminution of temperature on to the highest latitudes, there would be no winter. In short, the days and nights would be every where of the same length, except immediately around the poles, which would be always illuminated, and there would be no variety of seasons, but an unceasing verdure would everywhere prevail. And that the earth may have frequently passed through such a state, is highly probable from the fact, that at the present moment, the axis of Jupiter has very little if any inclination. And that this inclination has an extensive range of variation, would further ap pear from the fact, that in the case of Saturn it is now 28° 40', while in that of Mars it is 30° 18', according to Sir John Herschel. There is therefore no obvious reason why the inclination of the earth's axis should not go on augmenting until it arrives at the same angle.*

* In reality, it is more in accordance with analogy to suppose that this variation may pass through an entire revolution, than that it is confined within the narrow limits of 2o or 3°. Should such a revolution go on uniformly at the rate of 48" in a century, its completion would require a period of 2,700,000 years,-unless interrupted by a conjunction of all the planets; an event which the ancients regarded with dread, as the cause of deluges, or some other signal catastrophe. We are informed by Mr. Samuel Davis, that, according to the Surya Siddhanta, the oldest Hindoo work on Astronomy, the obliquity of the ecliptic was 24°, when it was written; which must therefore have been 4000 years before the 19th century. (Asiatic Researches, vol. ii.)

But are we not further authorized to conclude, that the equator may have gradually shifted to the middle and even the polar latitudes? This hypothesis would enable us to explain the high and uniform temperature which prevailed throughout the northern hemisphere when the higher latitudes abounded with tropical plants and animals,— a great physical fact which cannot be satisfactorily accounted for in accordance with the theory of Laplace. The truth is, that his mathematical reasonings were founded on the supposition that all the planets move in nearly the same plane; whereas it is now admitted by astronomers, that the orbit of Pallas is inclined to the ecliptic at an angle of 344°, and that of two satellites of Uranus at an angle of 78° 58'. But these cosmical bodies had not been discovered when the Mecanique Celeste was written; nor had Geology then taken its legitimate rank among the Sciences.

This brings us to the general theory of planetary motion. The leading facts of the solar system which connect the movements of the heavenly bodies with the agency and laws of caloric, may be reduced to the following propositions.

1. That the sun is 882, 000 miles in diameter, and revolves on his axis from west to east, in about twenty-five days.

2. That planets revolve around the sun, satellites around planets, and all of them upon their