374 Mr. J. Gill on the Dynamical Theory of Heat. 



imagined probably to extend ? The retardation of Encke's comet 

 is generally attributed to the resistance of the interstellar aether ; 

 but it is not improbable that the extension of the solar atmosphere 

 may play an important part in producing the effects observed. 

 The meteoric theory of the sun's heat has much probability in its 

 favour. The immense number of meteors which are periodically 

 seen from some parts of the earth's surface would seem to prove 

 the existence of an incalculable crowd of small asteroids circula- 

 ting round the suu. Moving through a resisting medium, these 

 small masses may be naturally supposed to be continually suffer- 

 ing a loss of projectile force or vis viva of orbital motion, and 

 consequently to be gradually approaching nearer and nearer to 

 the sun. The zodiacal light is supposed to owe its existence to 

 these crowded meteoric masses. " However this may be, it is 

 at least proved that this phenomenon arises from matter which 

 circulates in obedience to planetary laws ; the entire mass con- 

 stituting the zodiacal light must be continually approaching, and 

 incessantly raining its substance down upon the sun "*. The 

 heat produced by the collision of an incombustible asteroid thus 

 falling into the sun, has been calculated to be equal to 9000 

 times the heat generated by the combustion of an equal asteroid 

 of solid coal. Thus in the force of gravitation we perceive an 

 agency competent to maintain at the surface of the sun a tem- 

 perature far surpassing anything we can conceive of terrestrial 

 combustion. Helmholtz has shown that the force of gravitation 

 now existing as potential energy in the principal bodies of the solar 

 system might, if converted into heat, raise the temperature of a 

 mass of water equal to the sun and planets in weight 28 millions 

 of degrees Centigrade. From the incomparable penetrative 

 power of the solar rays, it is inferred that the temperature of 

 their source must be enormous; and is it not probable that the 

 almost inconceivable temperature of the fiery ocean forming the 

 sun's surface should volatilize the meteoric matter continually 

 falling into it, reducing it to a state of gaseous tenuity far 

 beyond anything we can perceive in terrestrial pheuomena, 

 and that from this vast alembic the subtle though still ponder- 

 able matter of the sun's atmosphere should be emitted in a con- 

 tinuous stream radiating into space ? It has been supposed by 

 eminent physicists that gaseous particles fly in straight lines 

 through space ; and if the atomic projectiles from the sun's sur- 

 face be supposed to be uninterrupted in their outward pro- 

 gress, they may be imagined to move with a velocity which, in 

 combination with the outward impulsion of the excited aether, 

 should carry them with gradually diminishing motion to an im- 

 mense distance into space (probably far beyond the orbit of the 

 * Prof. Tyndall. 



