1867.] Constitution of the Sun and Stars. 31 



major of the orbit of the meteors lies just along the line in which its plane 

 intersects the plane of the sun's equator. It is also very remarkable that the 

 interval of time in which the spots go through their mutations, which we 

 must of course adopt as the periodic time of the meteors in their orbit, 

 ■assigns to them an aphelion distance outside and close to the orbit of one of 

 the principal planets, Saturn. There is therefore very considerable ground 

 to suspect that there is such a swarm of meteors which was diverted into 

 the solar system by Saturn* at no very remote epoch — just as our 

 November meteors were brought in by the planet Uranus in the year 126 

 of the Christian era. 



Finally, it is shown that an hypothesis which has found much and de- 

 served favour of late years, that the heat expended by the sun is continually 

 restored to him by the falling in of meteors which had been circulating- 

 round him, is no longer tenable. 



The second part of the memoir treats of other stars. The differences in 

 their appearances are found to depend mainly on differences in the force of 

 gravity exerted at their surfaces. Where gravity on a star is feebler than 

 on the sun, either from the mass of the star being less, or from its being 

 so dilated by heat that its outer parts are further removed from its centre, 

 gases which by reason of the mass of their molecules are imprisoned within 

 the photosphere of the sun, will, when less attracted downwards, be able to 

 stand the coolness of the shell of clouds and pass beyond them. Thus 

 mercury, antimony, tellurium, and bismuth, all of which have too high a 

 vapour- density to exist in the sun's outer atmosphere, show themselves in 

 that of Aldebaran. Again, in these stars all the gases of the outer atmo- 

 sphere expand until their upper layers, those from which their spectral 

 lines issue, are cooler than on the sun. These spectral lines will accord- 

 ingly be darker than on the sun, and as this will tell with most effect on 

 the blue end of the spectrum, it will render the light from these stars ruddy. 



On the other hand, those stars which, either from being of greater mass 

 than the sun, or from being less hot in their internal parts, attract down 

 the gases of their outer atmospheres with more force, constitute the class 

 of intensely white stars with a somewhat violet tinge, of which Sirius and 

 a Lyree are examples. Several of the substances which in the sun's spec- 

 trum give rise to faint lines, are on such stars confined within the photo- 

 sphere ; and the lowest temperature which others of them can withstand, 

 is by reason of the force with which they are attracted downwards, hotter 

 than the corresponding temperatures of the sun. Hence the substances 

 which on the sun cause his numerous dark lines — sodium, magnesium, cal- 

 cium, chromium, manganese, iron — produce in the spectrum of the star 



* The attraction of Jupiter would also have been competent to divert a cluster of 

 meteors into an orbit of the requisite form and dimensions ; but the situation of the 

 orbit would in tbat case have caused the meteors to cross the path of Jupiter, so that the 

 planet would have acted ever since as a powerful dispersing agent, and it does not seem 

 likely that such an influence has been in operation. 



