Mechanical Energies of the Solar System. 425 



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 wiihin a fraction of 

 the sun^s radius from his surface, before falling in, if it continued 

 solid ; but we may be sure that it w^ould 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 evapo- 

 ration with so high a temperature). Even the planet Mercury, 

 if the sun is still bright when it falls in, will, in ail probability, 

 be dissipated in vapour long before it reaches the region of in- 

 tense resistance, instead of (as it would inevitably do if not vola- 

 tile) 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 sur- 

 face, 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 volatilized, 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, fail into 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 evapo- 

 ration, and afterwards partially emitted in their condensation at 

 a higher temperature, is probably as insensible, in comparison 

 with the heat of friction, as it has been shown the heat of any 



* 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 I'or 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 me- 

 teors' the smallest fragments may be wholly evaporated immediately; but 

 all whose dimensions exceed some very moderate 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 sur- 

 faces 



Phil. Mag. S. 4. Vol. 8. No. 54. Dec. 1854. 2 F 



