different degrees of velocity; and this motion appears to have had considerable influence on their external shape, by enlarging their equatorial diameters; they are not perfect spheres, but are more or less flattened at their poles.

In the planet Jupiter, the velocity of the equatorial parts is more than four hundred miles per minute, whilst in the same time the equatorial parts of the Earth have moved only seventeen miles. A difference between the polar and equatorial diameter of Jupiter is perceptible with a telescope that has a distinct magnifying power of a hundred times, and it is ascertained to be as 12 to 13. The equatorial diameter of the earth exceeds its polar about twentyseven miles; the length of the equatorial diameter being 7927, that of the polar 7900 miles.

The relative density of the sun, the earth, and of the other planets, is estimated by the attractive force which they exert on each other, as they move round their common centre of gravity. The absolute density or the quantity of matter contained in the earth, compared with an equal bulk of any known substance, may be nearly determined by the attractive force which any given mass of matter exerts upon a plummet (when suspended in its vicinity) to draw it from a vertical line. This will be proportional to the absolute quantity of matter in that mass compared with that of the earth. By this method, it has been found that the mean density of the earth is about five times greater than that of water, or nearly twice the average density of the rocks and stones on the surface.

Hence it may be inferred that the interior part

of the earth is solid; or, if it be cavernous, the solid matter must possess great density. It is not improbable that iron, nearly in a metallic state, may be one of the constituent parts of the central mass, and to this it may owe its magnetic polarity.

Dr. Halley has written a very ingenious paper in the Philosophical Transactions, to prove that the earth is a hollow sphere, in which there is inclosed a central magnetic globe, and by the motions of this globe the variations of the magnetic needle are produced. The celebrated French astronomer Laplace asserts that the nutation of the earth's axis, and experiments on the vibration of the pendulum, indicate that the mineral beds of which our planet is composed, increase in density as they approach nearer to its centre, at least to a certain depth from its surface. There are, however, terrestrial phænomena, such as the rapid transition of motion to very distant parts of the earth's surface during violent earthquakes, which would render probable the existence of interior cavities filled with fluid or gaseous matter, and extending from one quarter of the globe to the other, and from the lower parts, to near the surface. It is evident that we have no means of verifying or invalidating hypotheses respecting the nature and structure of the central parts of the globe. The matter thrown up from vast and unknown depths by subterranean fires is similar to that of many rocks on the surface; but we know not what changes it has undergone, or what substances were separated from it by fusion.

It is, however, the study of the crust of the globe that is the proper occupation of the Geologist; and

the greatest depth to which he can extend his observations from the uppermost strata, to the very lowest beds that have been raised up or laid bare by these natural operations which have formed mountains or valleys, is less than eight miles ; a thickness which, compared with the bulk of the earth itself, does not exceed that of a coat of varnish upon an artificial terrestrial globe. Were we to bear this sufficiently in mind, the mighty catastrophes which have changed the surface of the globe in former periods, and have left traces of their action, appalling to the imagination, would cease to exceed the sober measure of belief.

The superficies of our planet is calculated to contain about one hundred and ninety millions of square miles; but could we be raised to a sufficient height above the earth, so as to have its whole enlightened hemisphere for our horizon, we might perceive as it revolved under our feet, how small a portion is fitted for the habitation of man. More than three fifths of the earth's surface are covered by the ocean; and if from the remaining part we deduct the space occupied by polar ice and eternal snow, by sandy deserts, steril mountains, marshes, rivers and lakes, the habitable portion will scarcely exceed one fifth of the whole of the globe. Nor have we reason to believe that at any former period, the dominion of man over the earth was more extensive than at present. The remaining four fifths of our globe, though untenanted by mankind, are for the most part abundantly stocked with animated beings, that exult in the pleasure of existence, independent of human controul, and no way sub

servient to our necessities or caprice. Such is and has been for several thousand years the actual condition of our planet; nor is the consideration foreign to our subject, for hence we may feel less reluctance in admitting the prolonged ages or days of creation, when numerous tribes of the lower orders of aquatic animals lived and flourished, and left their remains imbedded in the strata that compose the outer crust of our planet.

The ocean has been an important agent in effecting vast changes on the surface of our globe, which will be afterwards considered. The average depth of the sea has been differently estimated; according to Laplace this depth cannot be less than ten miles, to account for the height of the tides by the laws of gravitation. No admeasurement by soundings has exceeded the depth of one mile and a quarter.

The ocean has not always been confined in its present bed, for rocks almost entirely composed of the shells or remains of marine animals, are found in almost every country that has yet been explored, and these remains occur near the summits of the highest mountains, in the old and new continents, some of which rise more than two miles above the present level of the sea.

It is well known that the water of the sea contains a considerable portion of common salt, and a small portion of other saline ingredients *. The average amount of salt in the ocean may be esti

*The inquiry has often been made,-whence did the sea derive its saline contents ? It has been supposed by some writers that

mated at 2 per cent of common salt and per cent of other saline compounds.

The atmosphere which surrounds the earth does not come under the attention of the geologist, except as an agent in wearing down the surface by moisture and change of temperature.

The inequalities of the earth's surface formed by mountains and valleys, afford frequent opportunities for observing that the mineral substances of which it is composed, are of different kinds: in some situations we observe strata of chalk; in others, of sandstone or limestone, or beds of slate, granite, &c. It was long known to working miners, that the different beds of mineral matter lay over each other in a regular order in certain districts, and that certain beds were always found under and never above other particular beds.

The first geological observation which may be said to have laid the foundation of the science, was made by the German Lehman. He found that the lower rocks in some of the mining districts were distinguished from the upper rocks by their great hardness and by their structure, which was for the most part either crystalline or slaty; they were also distinguished by the absence of shells and other

the salt in the sea has been gradually augmented by saline particles brought into it by rivers; but this cause is totally inadequate to explain the immense quantity of salt existing in the whole mass of the ocean. If the average depth of the sea be ten miles, and it contain 24 per cent of salt,—were the water entirely evaporated, the thickness of the saline residue would exceed one thousand feet; and were this removed and spread over all our present continents, it would form a stratum of solid rocksalt more than half a mile in thickness.