attraction with that of the mountain. From such data it was found that the mean density of the earth was to that of the mountain as 9 to 5; and from hence it was concluded that the interior of the globe must be composed of substances, whose density was about double that of the mass of the mountain. But it was subsequently found that the specific gravity of the mountain, an item in these calculations, had been estimated too high, and by the corrections made by Mr. Playfair, the density of the earth obtained in this way was found to be only 5. That is, the whole earth, bulk for bulk, is five times the weight of water. This estimate, which is most generally received at the present day, makes it necessary to suppose that the interior of the earth is much more dense than its surface; for the heaviest rock with which we are acquainted has a density of only 3, and the lightest about 24, while the specific gravity of the ocean is a little more than 1. The specific gravity of the earth's surface including the water, therefore cannot be much above 2. But the mean density of the whole earth being 5, is more than double that of its surface, and hence the interior of the earth must have a greater density than 5, to counterbalance this want of weight at the surface.

From these considerations, it has been supposed that the interior of the earth, instead of being composed of rocks, like the surface, must consist of metallic substances. It has also been shown from mathematical calculations that there is a gradual increase of density towards the centre of the earth, and hence it has been thought still more probable that its nucleus is of a metallic nature.

La Place, with this view of the earth's structure, has estimated its density at the centre. If 5, 4 be taken as its mean density and its superficial densities be assumed as 3, 13; 3, 2; 2, 79; and 2, 60, then on the theory of compressibility, the density at the centre will be 13, 25; 14, 54; 15, 78; and 20, 10, respectively. The least of these is about double the density of iron and the greatest exceeds that of gold, being about equal to that of hammered. platina, the most ponderous of all known substances. But both philosophy and conjecture are alike useless on this subject, for in spite of both, we must remain entirely ignorant concerning the composition of the earth's

centre.

Distribution of Sea and Land. Nearly three fourths of the whole surface of the globe is covered by water. The surface of the Pacific Ocean alone, is estimated to be somewhat greater than all the dry land with which we are acquainted.

The greatest elevation of land is about 25,000 feet above the level of the sea; but its greatest depression, being concealed by the water, cannot be ascertained, and hence the quantity of water which the oceans contain, cannot be estimated with any degree of accuracy. La Place, however, has made a computation of the mean depth of the sea, founded on the theory of the tides, by which he concludes that it is about twelve miles. Concerning the bottom of the sea, we know little more than that it consists of mountains and valleys, like the surface of the land. This is ascertained by the rocks, and islands which rise above the surface of the water, and the reefs and deep water, which are known to exist alternately, below it. The sounding line of the mariner not only detects this unevenness of the submarine surface, but also the steep acclivities of its mountains and the gradual risings of its sand banks.

Composition and specific gravity of the Sea. The whole ocean is composed of salt water, though it varies considerably with respect to the quantity of solid matter it contains. At the mouths of rivers, and in bays which receive streams from the land, it is obvious that the water cannot be so fully saturated with salt, as it is where the sea is not thus diluted. There is also a difference with respect to the quantity of salt which different oceans contain, independently of any such circumstance. This is ascertained, not only by the analysis of their waters, but also by their different specific gravities.

It will be remembered that the standard, or unity, by which specific gravities are estimated, is distilled water, which is 1; and therefore the greater the quantity of solid matter any water contains, the greater will be its specific gravity.

Dr. Marcet instituted a series of experiments on sea water from different parts of the world, from which he obtained the following results and conclusions.

1. "That the Southern Ocean contains more salt than the Northern, in the ratio of 1.02919, to 1.02757.”

2. "That the mean specific gravity of sea-water, near

the equator, is 1.02777, intermediate between that of the northern and southern hemispheres."

3. "That there is no notable difference in sea-water, under different meridians."

4. "That there is no satisfactory evidence that the sea at great depths, is more salt than at the surface."

5. "That the sea, in general, contains more salt where it is deepest and most remote from land; and that its saltness is always diminished in the vicinity of large masses of ice."

6. "That small inland seas, though communicating with the ocean, are much less salt than the ocean itself.”

7. "That the Mediterranean contains a larger proportion of salt than the ocean."

Temperature of the Earth. The superficial temperature of the earth, if not entirely due to the heat of the sun, is greatly influenced by it. Still, local circumstances cause considerable variations in different places situated under the same latitudes. These circumstances will be noticed in their proper place.

Geological investigations have proved that the temperature of the earth has not always remained the same; but that the climates of different countries, and probably the superficial heat of the entire globe have greatly deteriorated, since the time when the elephant inhabited Siberia, and the mastodon, the forests of North America. This subject will be examined under the articles " Change of Climate," and " Organic Remains."

With respect to the internal temperature of the Earth, the prevailing opinion among geologists of the present day, appears to be that the heat increases in some proportion to the distance of descent from the surface. That this is the case, seems to be proved by the experiments made in mines, situated in different parts of the world, an account of which will be given hereafter.

Temperature of the Sea and of Lakes. The maximum density of fresh water is at the temperature of 40 degrees of Fahrenheit, and it has been considered that sea-water follows a similar law of condensation. Now water being free in its motions, arranges itself according to its density; that which is at, or near the temperature of 40°, occupy

ing the lowest place, while that which is warmed by the sun, is superincumbent on this.

In 1819 and 1820, Mr. De la Beche made numerous experiments, with great care, on the temperature of the Swiss lakes, and from which he found that between the surface and the depth of 40 fathoms, there was a material variation of temperature. From one to five fathoms, in the month of September, the heat was from 64° to 67°; but below this, the temperature decreased down to 40 fathoms. From 40 to 90 fathoms, the thermometer stood almost uniformly at 44°; and from 90 to 164 fathoms, it invariably stood at 43°.5.

In the winter, these experiments were repeated, and it was found that the temperature of the water followed the same law.

The temperature of the sea at different depths, accords sufficiently well with the observations already made; the temperature diminishing to the depth where the fluid attains its greatest density, below which it remains the same, or at a similar temperature. It appears, however, that there is considersble difference in the temperature of different seas at similar depths. Thus Capt. Kotzebue, in latitude about 36° N. and longitude 1480 W. when the surface of the water was nearly 739; found the temperature 57°, at 25 fathoms; 52.8, at 100 fathoms; and 440 at 300 fathoms. While the same observer in lat. 30° 39' S. found a temperature of 49° 5' at 35 fathoms; and in a similar latitude S. 38° 8' in 196 fathoms. It will be observed, however, that the same law is maintained, both in salt and fresh water, viz. a decrease of temperature downwards. But this fact is not at variance with the probability of an internal, or central heat, since the waters arrange themselves in the order of their densities, and this would take place, whether the bottoms of deep seas were cold or warm.

Temperature of the Atmosphere. The atmosphere is composed of two gaseous substances, called oxygen and nitrogen, and in the proportion of 20 parts of the first, to 80 of the last. From its refractive powers, it has been calculated that the atmosphere reaches to the height of about 45 miles above every part of the earth.

The heat which is constantly radiating from the earth, is absorbed by the atmosphere, so that its temperature in hot climates often exceeds that of the human system. That

the temperature of the air is dependent on the heat of the earth's surface, is proved by the well known fact, that it constantly diminishes as we ascend upwards, or recede from the earth. Hence in the hottest climates, there is a region a few thousand feet above the earth, to which its heat never ascends in such quantity as to prevent perpetual congelation.

The line of perpetual snow, we should suppose would differ in elevation, (under equal circumstances,) according to the distance from the equator. It is, however, liable to considerable variations, probably from local cau

ses.

The following table, from Encyc. Brittanica, article Climate, presents the different elevations at which there is constant frost, under different latitudes.

From this table, we learn that there is no regular correspondence between the latitude and the height of perpetual frost, and that the difference in this respect is much greater than might have been expected from the influence of local causes. Thus the difference between the freezing height at the equator, and in latitude five degrees, is only one hundred and twelve feet; though at the other extreme, from eighty to eighty-five degrees, this difference is upwards of three hundred feet. Much the greatest difference is in the temperate latitudes, as between thirty-five degrees and forty degrees, where the elevation is from 10,287, down 9,001, making a difference of 1,286 feet in five degrees. Whether these differences are entirely dependent on local causes, we have no means of deciding.

In the elevation of mountains to the region of perpetual