periphery of this wheel against something in surrounding space which produces the light and heat? Such a notion has been entertained. But what forms the brake, and by what agency is it held, while it rubs against the sun? The action is inconceivable; but, granting the existence of the brake, we can calculate the total amount of heat which the sun could generate by such friction. We know his mass, we know his time of rotation; we know the mechanical equivalent of heat; and from these data we deduce, with certainty, that the entire force of rotation, if converted into heat, would cover more than one, but less than two, centuries of emission. There is no hypothesis involved in this calculation. "There is another theory, which, however bold it may at first sight appear, deserves our earnest attention. I have already referred to it as the meteoric theory of the sun's heat. Solar space is peopled with ponderable objects. Kepler's celebrated statement that 'there are more comets in the heavens than fish in the ocean' refers to the fact that a small portion only of the total number of comets belong to our system, and are seen from the earth. But besides comets, and planets, and moons, a numerous class of bodies belong to our system--asteroids, which from their smallness might be regarded as cosmical atoms. Like the planets and the comets these smaller bodies obey the law of gravity, and revolve in elliptic orbits around the sun; and it is they, when they come within the earth's atmosphere, that, fired by friction, appear to us as meteors and falling stars. On a bright night twenty minutes rarely pass at any part of the earth's surface without the appearance of at least one meteor. At certain times (the 12th of August and the 14th of November), they appear in enormous numbers. During nine hours of observation in Boston, when they were described as falling as thick as snowflakes, 240,000 meteors were calculated to have been observed. The number falling in a year might perhaps be estimated at hundreds or thousands of millions, and even these would constitute but a small portion of the total crowd of asteroids that circulate round the sun. From the phenomena of light and heat, and by the direct observation of Encke, on his comet, we learn that the universe is filled with a resisting medium, through the friction of which all the masses of our system are drawn gradually toward the sun. And though the larger planets show, in historic times, no diminution of their periods of revolution, this may not hold good for the smaller bodies. In the time required for the mean distance of the earth from the sun to alter a single yard, a small asteroid may have approached thousands of miles nearer to our luminary.