428 Professor W. Thomson on the 



25,000 years is the time the sun would take to acquire his 

 actual motion of rotation, by the incorporation of meteors, if 

 these bodies were each revolving in the plane of his equator im- 

 mediately before entering the region of intense resistance. But 

 it has been shown to be probable that a great space round the 

 sun is occupied by a vortex of evaporated meteors, and that the 

 incorporation of meteoric matter takes place in reality by the 

 condensation of vapour in a stratum close to his surface all 

 round. It appears not improbable that the tangential velocity 

 of this vortex immediately external to the radiant region of 

 intense resistance may be found to be, in all solar latitudes, very 

 nearly that of a planet close to the sun. If it be so, the moment 

 of the motion communicated to the sun by any mass of meteoric 



matter will be ."' of what would be communicated 



4 



by the incorporation of an equatorial planet of equal mass : as 

 much as 3^Q of the sun's mass would have to fall in to produce 

 his present rotation : and 32,000 years would be the time in 

 which this would take place, at the present rate of meteoric 

 incorporation as estimated above. 



It will be a very interesting hydrodynamical problem to fully 

 investigate the motion of the meteoric vortex; and among 

 results to be derived from it will be strict estimates of the con- 

 tribution to the sun's rotatory motion, and of the quantity of 

 heat generated, by any amount of meteoric matter in becoming 

 incorporated. With these, and with an accurate determination 

 of the rate at which the sun radiates heat, we should be able to 

 fix with certainty the augmentation of his velocity of rotation 

 actually taking place at present from year to year, and to esti- 

 mate the time during which the existing rotation would be 

 acquired by meteoric incorporation going on always at the pre- 

 sent rate and in the present manner. Whatever this time 

 (which I shall call T years, to avoid circumlocution below) may 

 be, probably will not be found to differ very widely from the 

 preceding estimate of 32,000 years. 



Now, from the fact that the sun's equator, the planets' orbits, 

 and the zodiacal light, all lie nearly in one plane, it appears 

 highly probable that the sun's present motion has really been 

 acquired by the incorporation of meteors. It is certain that the 

 present manner and rate of meteoric action cannot have been 

 going on for more than the indicated period (T), without giving 

 the sun a greater rotatory motion than he has, unless (which is 

 very improbable) he were previously rotating in a contrary direc- 

 tion round the same axis : and, at only the present rate, it 

 cannot have been going on for less than that time, unless the 

 sun has been created with a rotatory motion round his present 



