Vi.] THE MOTIVE POWERS OF THE UNIVERSE. 61 



thousand times its weight of coal ; and if the quantity of 

 heat constantly radiated by the sun were supplied by the 

 combustion of the sun's substance, supposing the sun to be 

 of coal, and oxygen to be supplied from without, it would 

 be burned away in about 26,000 years ^ at the most. 



I ought perhaps to repeat, that meteoric heat is the Meteoric 

 energy of motion of the meteors, which is transformed into *' 

 heat in the act of falling into the atmosphere of a sun or a 

 planet. The intensely bright light of the meteors that 

 flash across our sky shows how hot they must be ; and 

 in consequence of the sun's greater attractive force, the 

 meteors fall into his atmosphere with a very much higher 

 velocity than into ours, and, of course, produce heat in the 

 ratio of the square of the velocity. 



But, however strong may be the proof of the meteoric Infinite 

 theory of solar heat, it does not prove the supply of solar ^rteors"^ 

 heat to be inexhaustible; for an endless supply of heat 

 would need an infinite supply of meteors to yield it, and 

 an infinite supply of meteors could only be drawn from 

 infinite space. Such a supply may possibly be brought in ; possible, 

 there is nothing in the supposition in contradiction with "* 

 the laws of force ; but if, in the course of ages to which 

 all historical time is but a point, vast masses of meteoric 

 matter fall in from regions beyond the orbits of the planets, 

 and increase the sun's mass and consequently his attractive 

 power, this increase of the sun's attractive power, not would 

 being balanced by any increase in the centrifugal force fh^^'' u- 

 of the motions of the planets, will ultimately cause the librium of 

 planets themselves to fall into the sun. sYstem^ 



Consequently, one of two alternatives must be true. If Two alter- 

 the supply of meteors is limited, the sun's heat will be exbTustioa 



1 "Were, the heat radiated away by the sun supplied by the combustion 

 of coal, it would require the combustion every year of a layer of coal about 

 seventeen miles thick all over the sun's surface. (Tyndall on Heat, p. 419.) 

 As the sun's radius is about 441,000 miles, this would involve the com- 

 bustion of his whole substaiice in about 26,000 years. On this supposition, 

 however, the sun's mass, and consequently his surface, would be constantly 

 growing less, and would radiate less and less heat ; but were the supply 

 by any means to be kept up at the present rate, the sun would be burned 

 out in a much shorter time. 



