426 Professor W. Thomson on the 



combustion or chemical action they can experience must be, or, 

 as we have tacitly assumed the heat is which is taken and kept 

 by the meteors themselves in approaching from cold space to 

 lodge permanently in the sun. We may conclude that the sun's 

 heat is caused, not by solids striking him, or darting through 

 his atmosphere, but by friction in an atmosphere of evaporated 

 meteors, drawn in and condensed by gravitation while brought 

 to rest by the resistance of the sun's surface. The quantity of 

 meteoric matter required, if falling in solid, would, as we have 

 seen, be such that half the work done by solar gravitation on it, 

 in coming from an infinite distance, is equal to the energy of 

 heat emitted from the sun, and would therefore amount to a 

 pound every 2*3 hours per square foot of the sun's surface ; and 

 it will be the same as this, notwithstanding the process of eva- 

 poration and condensation actually going on, if, as appears pro- 

 bable enough, the velocity of the vortex of vapour immediately 

 external to the region of intense resistance in all latitudes be 

 nearly equal to that of a planet close to the sun. 



No. III. On the Distribution of Temperature over the Sun's 

 Surface. 



Not only the larger planets, but the great mass of meteors 

 revolving round the sun, appear to- revolve in planes nearly co- 

 inciding with his equator, and therefore such bodies, if solid 

 when drawn in to the sun, would strike him principally in his 

 equatorial regions, and would cause so much a more copious 

 radiation of heat from those regions than from any other parts 

 of his surface, that the appearance would probably be a line or 

 band of light, instead of the round bright disc which we see. 

 The nearly uniform radiation which actually takes place from 

 different parts of the sun's surface appears to be sufficiently 

 accounted for by the distillation of meteors, which we have seen 

 must, in all probability, take place from an external region of 

 evaporation at a considerable distance (perhaps several times his 

 radius) inwards to his surface where they are condensed. What- 

 ever be the dynamical condition of the luminous atmosphere of 

 intense resistance, it is clear that there must be a very strong 

 tendency to an equality of atmospheric pressure over the pro- 

 bably liquid surface of the sun, and that the temperature of the 

 surface must be everywhere kept near that of the physical equi- 

 librium between the vapours and the liquid or solid into which 

 they are distilling. A lowering of temperature in any part would 

 therefore immediately increase the rate of condensation of vapour 

 into it, and so bring a more copious influx of meteoric matter 

 with dynamical energy to supply the deficiency of heat. The 

 various deviations from uniformity which have been observed in 



