in an opposite direction, making the angle of reflection equal to the angle of incidence.

When rays of light proceed in all directions from a luminous body, they are said to diverge; and when, as by a concave mirror, they are collected together at one point, they are said to converge, and that point is called the focus, where are assembled the whole particles of light and heat reflected from the concave surface of the mirror, which, from this circumstance, is commonly called a burning glass.

From observing the effects produced on the rays of light, by their transmission through a medium whose opposite surfaces are convex, or portions of spheres swelling outwards, common spectacles and telescopes have been constructed; some of these last possessing the power of enlarging the images of distant objects to a very high degree.

Although light appears in general white, yet it is composed of rays of various colours, as red, orange, yellow, green, blue, purple, and violet, as may be observed by transmitting a ray of the sun's light into a dark room, through a prism of pure glass; for the different parts of rays being susceptible of different degrees of refraction, these parts will be separated in passing through the prism, and exhibit on a sheet of paper the colours peculiar to each part of the white ray. It is by the refraction of the sun's rays in passing through the minute globules of which vapours and clouds consist, that the beautiful arch in the heavens, to which we give the name of the rainbow, is produced.

It was before noticed, that light and heat are by no means inseparable heat is a substance by whose action fluids are converted into vapour, and solids are either converted into vapour, rendered fluid, changed into glass, or undergo other processes observable in a state of combustion. By light we acquire the sensation of vision; by heat that of warmth, or of heat itself. By heat bodies are expanded or enlarged;

VOL. II.

and

and hence we have the instrument called the thermometer, for measuring the quantities of heat existing in bodies, agreeably to the import of the Greek words composing the name; in the same way as the instrument for measuring the relative weights of the air is called a barometer.

The sensation of heat is produced in our body by particles of heat entering it; and the sensation of cold is excited by such particles passing out of the body. Heat seems to be the chief cause of the fluidity of bodies: wax, lead, silver, &c. become fluid by the application of heat; and oil, water, even mercury itself, are rendered solid by being deprived of heat.

IN general it may be observed, that the highest ranges of mountains on the surface of the earth are situated between the tropics, and that their elevation diminishes as their pɔsition approaches to the poles. The loftiest parts of the grand chain or cordillera of the Andes, in South America, lie under the equator, one of the summits, Chimborazo, springing up to no less a height than 20,608 English feet, while the highest mountains in North America do not exceed 4000 feet.

The following Table shows the height of some of the most remarkable mountains in the world, whose elevation has yet been accurately ascertained. The second column gives the elevation as determined by means of the barometer; the third column, that discovered by geometrical operations and when the methods by which the measurements were performed are unknown, the results are entered in the fourth column,

TABLE.

The sea or ocean covers about two thirds of the surface of the globe; but the quantity of water it contains will perhaps never be ascertained, from our ignorance of its various depths. The greatest depth which has ever been measured is 5,346 feet, or a little more than an English mile; and its mean depth has been calculated at about 1,300 feet, or a quarter of a mile. At great distances from the land the depth of the sea increases or diminishes with some regularity, but near the shore it varies very considerably; bearing, however such a relation to the land, that along a high steep shore the water deepens very rapidly, while before a low flat beach, it deepens very slowly for a good way out.

:

If the whole inhabited parts of the earth be supposed to contain 38,990,569 square miles, Europe will contain 4,456,065, Asia 10,768,823, Africa 9,654,807, and America 14,110,874 and if the whole population be estimated at 700,500,000 persons, Europe will possess about 150,000,000; Asia, including the great country of New Holland and the isles in the Pacific Ocean, 500,500,000; Africa 30,000,000, and America 20,000,000: so that the population on a square mile, at an average, will be in Europe 34, in Asia 46, in Africa 3, and in America 14, or 3 for every 2 square miles.

Before