Mechanical Energies of the Solar System. 429 



axis, or has acquired such a motion from some independent 

 mechanical action. The actual rate of solar radiation in time 

 past may, for all we know, have been sometimes much greater 

 and sometimes much less than at present ; and there probably 

 has been a time before, when meteors in abundance fell direct 

 to the sun from extra-planetary space, some getting stopped on 

 their way by the earth, and illuminating it by friction in its 

 atmosphere and impact at its surface. But the kind of meteoric 

 action now going on, has in all probability produced neither 

 more nor less than T times the quantity of heat now emitted 

 from the sun in one year. All things considered, it seems not 

 improbable that the earth has been efficiently illuminated by the 

 sun alone for not many times more or less than 32,000 years. 



As for the future, it will be a most interesting problem to 

 determine the mass of the zodiacal light (that is, matter external 

 to the sun^s mass, and within the earth^s orbit), by the pertur- 

 bations it may probably enbugh be discovered to produce in the 

 motions of the visible planets. It could scarcely, I think, 

 amount to j th 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 astronomical science; 

 and we have seen that meteors amounting to j^^^^dth 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 y^ of its present value. 



If T be 32,000, according to the preceding conjectural estimate, 

 the effect in fifty-three 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 possible 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 



Days. Hours. Minutes. 



* According to Bohm 25 12 30 



Laugier 25 8 10 



Petersen 25 4 30 



(See Encyc. Brit., Sth 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. 



