THE SUN'S HEAT. 23 



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 in- 

 candescence, and immensely more than could be produced by its com- 

 plete combustion under any conceivable circumstances. With refer- 

 ence 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 : 



Tears. Days. 



Mercury 6 219 



Venus.' 83 326 



Earth 95 19 



Mars 12 259 



Jupiter 32,254 



Saturn 9,652 



Uranus 1,610 



Neptune 1,890 



Total , 45,604 



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 there- 

 fore 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 improb- 

 able, for astronomical reasons. If so large a quantity of matter annu- 

 ally 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 



