grade; if it were moving 500 metres per second (about the speed of a cannon-ball), it would produce 250,000 times as much heat, or enough to raise the temperature of a mass of water equal to itself nearly 300° C. If it were moving, not 500 metres per second, but about 700,000 (approximately the velocity with which a body would fall into the sun from any planetary distance), the heat produced would be 1,400 x 1,400, or nearly 2,000,000 times as great—sufficient to bring a mass of matter many thousand times greater than itself to most vivid incandescence, and immensely more than could be produced by its complete combustion under any conceivable circumstances. With reference to this theory, Sir William Thomson has calculated the amount of heat which would be produced by each of the planets in falling into the sun from its present orbit. The results are as follows, the heat produced being expressed by giving the number of years and days through which it would maintain the sun's present expenditure of energy:
That is, the collapse of all the planets upon the sun would generate sufficient heat to maintain its supply for nearly 46,000 years.
A quantity of matter equal to only about one one hundredth of the mass of the earth, falling annually upon the solar surface, would therefore maintain its radiation indefinitely. Of course, this increase of the sun would cause an acceleration of the motion of all the planets—a shortening of their periods; since, however, the mass of the sun is 330,000 times that of the earth, the yearly addition would be only one thirty-three millionth of the whole, and it would require centuries to make the effect sensible. The only question, then, is whether any such quantity of matter can be supposed to reach the sun. While it is impossible to deny this dogmatically, it on the whole seems improbable, for astronomical reasons. If so large a quantity of matter annually falls upon the solar surface, it is necessary to suppose a vastly greater quantity circulating around the sun, between it and the planet Mercury. The process by which the orbit of a meteoric body is so changed as to make it enter the solar atmosphere is a very slow one; so that only a very small proportion of the whole could be caught in any given year. But, if there were near the sun any considerable quantity of meteoric matter—anything like the mass of the earth, for