882 RADIANT HEAT. 



not bo missed from the zodiacal light. Thus the sun's heat might be 

 kept up for five thousand years to come at least. This transfer of 

 matter to the sun coming from a source within the earth's orbits 

 would not affect the conditions of the system as to the effects of gravi- 

 tation. 



Some meteors may possibly come direct on the sun from the extra- 

 planetary spaces, but the quantity of such is probably very small in 

 comparison with those that have been revolving in approximately 

 circular or elliptic orbits before falling in. 



If we imagine a dark body moving through space and coming into 

 a locality abounding with meteors, their impact may raise it to incan- 

 descence, which will cease when it moves out of that space. Thus 

 the author suggests a possible explanation of variable stars. 



(Addition I.) The author gives a calculation of the quantity of 

 matter necessary to be added to the sun on the extra-planetary hy- 

 pothesis, and finds it gives too great an increase of central force to 

 consist with the historical conditions of the earth's motion. He con- 

 cludes the supply must have been from within the earth's orbit for 

 thousands of years at least. 



(Addition II.) He shows that the resistance must be very small 

 even close to the sun, since such light bodies as comets pass through 

 it at perihelion. 



The solar atmosphere may be conceived to be carried round in a 

 vortex by these revolving masses, but not more rapidly than a planet 

 would be at the same distance. Hence the meteors must long con- 

 tinue to revolve before reaching the sun, and must get so near as to be 

 completely evaporated before they fall in. 



Hence the solar heat is produced, not by solids impinging on the sun, 

 hut by the violent friction of tJie rotating vortex of evaporated meteoric 

 matter. 



(Addition III.) The temperature of the different parts of the sun's 

 surface may undergo great changes from the eddies and streams oc- 

 curring in this revolving mass. Hence many of the appearances of 

 the solar spots and streaks, &c. 



(Addition IV.) ' ' On the age of the sun." At the rate of meteoric 

 incorporation above calculated, the present rotation of the sun would 

 be produced from rest in thirty-two thousand years. We may infer 

 (since it appears very improbable that the sun has had a contrary 

 rotation destroyed by meteoric incorporation) that the kind of agency 

 now going on cannot have been going on and alone generating heat 

 at the present rate for more than that period. For the future we 

 know that the mass of the zodiacal light is small, in comparison with 

 that of the sun, from its producing no sensible perturbation on the 

 planets, and we may be sure it cannot keep up the supply for three 

 hundred thousand years. The sun's rotation has been by no means 

 accurately determined; it may possibly vary to an amount which fu- 

 ture observations may detect, and thus test the theory. 



