7(5 PROFESSOR W. THOMSON ON THE 



At one-seventh of the Sim's radius from his surface, this would be about 258 

 miles per second ; and, therefore, a comet approaching so near the Sun, could not 

 have a less velocity relatively to the resisting medium than 107 miles per second, 

 and, if going against the stream, might have as great a relative velocity as 623 

 miles. On the other hand, the great body of the meteors circulating round the 

 Sun, and carrying the resisting medium along with them, may be moving through 

 it with but small relative velocities ; the smaller for each individual meteor, the 

 smaller its dimensions. The effects of the resistance must, therefore, be very 

 gradual in bringing the meteors in to the Sun, even when they are very near his 

 surface : and we cannot tell how many years, or centuries, or thousands of years, 

 each meteor, according to its dimensions, might revolve within a fraction of the 

 Sun's radius from his surface, before falling in, if it continued solid ; but we may 

 be sure that it would so revolve long enough to take, in its outer parts at least, 

 nearly the temperature of that portion of space ; and therefore, probably, unless 

 it be of some substance infinitely less volatile than any terrestrial or meteoric 

 matter known to us, long enough to be wholly converted into vapour: (the mere 

 fact of a comet* escaping from so near the Sun as has been stated, being enough 

 to show that there is, at such a distance, no sufficient atmospheric pressure to 

 prevent evaporation with so high a temperature;. Even the planet Mercury, if the 

 Sun is still bright when it falls in, will, in all probability, be dissipated in vapour 

 long before it reaches the region of intense resistance ; instead of (as it would 

 inevitably do if not volatile) falling in solid, and in a very short time (perhaps a 

 few seconds) generating three years' heat, to be radiated off in a flash which 

 would certainly scorch one half of the earth's surface, or perhaps the whole, as 

 we do not know that such an extensive disturbance of the luminiferous medium 

 would be confined by the law of rectilineal propagation. Each meteor, when vola- 

 tilized, will contribute the actual energy it had before evaporation to a vortex 

 of revolving vapours, approaching the sun spirally to supply the place of the 

 inner parts, which, from moving with enormously greater velocities than the parts 

 of the Sun's surface near them, first lose motion by intense resistance, emitting an 

 equivalent of radiant heat and light, and then, from want of centrifugal force, fall 

 in to the Sun, and, consequently, become condensed to a liquid or solid state at his 

 surface, where they settle. The latent heat absorbed by the meteors in evaporation, 

 and afterwards partially emitted in their condensation at a higher temperature, is 



* That a comet may escape with only a slight loss by evaporation, if the resistance is not too 

 great to allow it to escape at all, is easily understood, when we consider that it cannot be for more 

 than a few hours exposed to very intense heat (not more than two or three hours within a distance 

 equal to the Sun's radius from his surface). If it consist of a cloud of solid meteors, the smallest 

 fragments may be wholly evaporated immediately ; but all whose dimensions exceed some very mo- 

 derate limit of a few feet would, unless kept back by the resisting medium and made to circulate 

 round the Sun until evaporated, get away with only a little boiled off from their surfaces. 



