24 THE POPULAR SCIENCE MONTHLY. 



annulled, so that there shall be perfect freedom for convection-currents 

 to flow unresisted in any direction, except so far as resisted by the 

 viscosity of the fluid, and leave the piece of matter, which we may 

 now call the sun, to himself. He will immediately begin showing all 

 the phenomena known in solar physics. Of course, the observer 

 might have to wait a few years for sun-spots, and a few quarter- 

 centuries to discover periods of sun-spots, but they would, I think I 

 may say, probably, all be there just as they are : because I think we 

 may feel that it is most probable that all these actions are due to the 

 sun's own mass and not to external influences of any kind. It is, how- 

 ever, quite possible, and indeed many who know most of the subject 

 think it probable, that some of the chief phenomena due to sun-spots 

 arise from influxes of meteoric matter circling round the sun. The 

 energy of chemical combination is as nothing compared with the 

 gravitational energy of shrinkage, to which the sun's activity is almost 

 wholly due, but chemical combinations and dissociations may, as urged 

 by Lockyer, be thoroughly potent determining influences on some of 

 the features of non-uniformity of the brightness in the grand phe- 

 nomena of sun-spots, hydrogen-flames, and corona, which make the 

 province of solar physics. But these are questions belonging to a 

 very splendid branch of solar science with which we are not occupied 

 this evening. 



What concerns us at present may be summarized in two proposi- 

 tions : 



1. Gigantic convection-currents throughout the sun's liquid mass 

 are continually maintained by fluid, slightly cooled by radiation, falling 

 down from the surface, and hotter fluid rushing up to take its place. 



2. The work done in any time by the mutual gravitation of all the 

 parts of the fluid, as it shrinks in virtue of the lowering of its temper- 

 ature, is but little less than (so little less than, that we may regard it 

 as practically equal to*) the dynamical equivalent of the heat that is 

 radiated from the sun in the same time. 



The rate of shrinkage corresponding to the present rate of solar 

 radiation has been proved to us, by the consideration of our dynamical 

 model, to be thirty-five metres on the radius per year, or one ten-thou- 

 sandth of its own length on the radius per two thousand years. Hence, 

 if the solar radiation has been about the same as at present for two 

 hundred thousand years, his radius must have been greater by one per 

 cent two hundred thousand years ago than at present. If we wish to 

 carry our calculations much farther back or forward than two hundred 

 thousand years, we must reckon by differences of the reciprocal of the 

 sun's radius, and not by differences simply of the radius, to take into 

 account the change of density (which, for example, would be three per 



* " On the Age of the Sun's Ileat," by Sir William Thomson (" Macmillan's Maga- 

 zine, March, 1862) ; and Thomson and Tait's " Natural Thilosophy," second edition, vol. 

 i, part ii, Appendix E. 



