424 Professor W. Thomson on the 



year. Similarly, we should find a loss of ^th of a year on a period 

 of 2000 years ago ; that is, of about a month and a half since 

 the Christian sera. Thus, if we reckon back about 2000 times 

 the number of days at present in the yeai*, we should find seasons, 

 new and full moons, and eclipses, a month and a half later than 

 would be if the year had been constantly what it is. Now we 

 have abundant historical evidence that there is no such disloca- 

 tion as this, either in the seasons, or in the lunar phsenomenaj 

 and it follows that the central attracting mass of the solar 

 system does not receive the augmentation required by the extra- 

 planetary meteoric theory of solar heat. But the reasoning in 

 the preceding paper establishes, with very great probability, a 

 meteoric theory of solar heat ; and we may therefore conclude 

 that the meteors supplying the sun with heat have been for 

 thousands of years far within the earth's orbit. 



No. II. FHction between Vortices of Meteoric Vapour and the 

 Sun's Atmosphere the immediate Cause of Solar Heat. 



It has been shown that the meteors which contribute the 

 energy for solar heat must be for thousands of years within the 

 earth's orbit before falling to the sun. But a meteor could not 

 remain for half a year there, unless it were revolving round the 

 sun, with, at each instant, the elements of a circular or elliptic 

 orbit. Hence meteors, on their way in to the sun, must revolve, 

 each, thousands of times round him, in orbits which, whatever 

 may have been their primitive eccentricities, must tend to be- 

 come more and more nearly circular as they become smaller, by 

 the efl'ects of the resisting medium. The resistance must be 

 excessively small, even very near the sun ; since a body of such 

 tenuity as a comet, darting at the rate of 365 miles per second, 

 within one-seventh of his radius from his surface, comes away 

 without sensible loss of energy. If, as is probable, the atmo- 

 sphere of that part of space is carried in a vortex round the sun 

 by the meteors and other planets, it may be revolving at nearly 

 the same rates as these bodies at different distances in the prin- 

 cipal plane of the solar system ; but we cannot conceive it to be 

 revolving in any locality more rapidly than a planet at the same 

 distance. At one-seventh of the sun'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 rela- 

 tively to the resisting medium than 107 miles per second, and, 

 if going against the stream, might have as great a relative velo- 

 city 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 



