80 ON THE MECHANICAL ENERGIES OF THE SOLAR SYSTEM. 



of the Zodiacal Light (that is, matter external to the Sun's mass, and within the 

 Earth's orbit), by the perturbations it may probably enough be discovered to pro- 

 duce in the motions of the visible planets. It could scarcely, I think, amount to 

 55 of the Sun's mass (probably not to nearly as much), without producing such 

 perturbations as could not have been overlooked in the present state of astrono- 

 mical science ; and we have seen that meteors amounting to 5^ of the Sun's 

 mass, must, at the present estimated rate, fall in in 3000 years. I conclude that 

 Sunlight cannot last as at present for 300,000 years. 



The continual acceleration of the Sun's rotatory motion, which the preceding 

 theory indicates, must, sooner or later, be tested by direct observation. The rate 

 of acceleration (which for many thousands of years past and to come must remain 

 sensibly constant, if the solar radiation continues so), is such that the angular 



velocity is increased annually by tjt of its present value. If T be 32,000, accord- 

 ing to the preceding conjectural estimate, the effect in 53 years would amount to 

 diminishing the period of the Sun's revolution by an hour ; and the actual effect 

 cannot, according to the theory, be incomparably greater or less. It is just pos- 

 sible that a careful comparison of early with recent observations on the apparent 

 motions of the dark spots may demonstrate this variation ; but as some of the 

 most accurate of recent observations of this kind have led to estimates of the pe- 

 riod of revolution* differing from one another by as much as 8 hours, it is more 

 probable that, unless some way be discovered for taking into account the motions 

 of the spots themselves with reference to the mass, centuries will elapse before 

 direct evidence can be had either for or against the anticipated acceleration of 

 the Sun's rotatory motion. 



* According to Bohm, 



„ Laugier, 



„ Petersen, . 



(See Encyc. Brit., 8tli edit. vol. iv., p. 87.) The discrepancies are probably due to proper motions 

 of the spots, which, from the explanation given above in Addition iii., may be expected to be very 

 considerable. 



Days. 



Hours. 



Minutes. 



25 



12 



30 



25 



8 



10 



25 



4 



30 



