6 Sir William Thomson [Jan. 21, 



red-hot on its upper surface, the whole pit full of fluid would go on 

 cooling with extreme slowness until, after possibly about a million 

 million million years or so, it would be all at the same temperature 

 as the space to which its upper end radiates. 



Let precisely what we have been considering be done for every 

 one of our pyramidal rods, with, however, in the first place, thin 

 partitions of matter impervious to heat separating every pit from 

 its four surrounding neighbours. Precisely the same series of 

 events as we have been considering will take place in every one of the 

 pits. 



Suppose the whole complex mass to be rotating at the rate of 

 once round in twenty-five days, which is, about as exactly as we know 

 it, the time of the sun's rotation about his axis. 



Now at the instant when the paddle stops let all the partitions 

 be annulled, so that there shall be perfect freedom for 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 sunspots, and a few quarter- 

 centuries to discover periods of sunspots, but they would, I think 

 1 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 substance, and not to external influences of any kind. It 

 is, however, quite possible, and indeed many who know most of the 

 subject think it probable, that some of the chief phenomena due 

 to sunspots 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. A body falling forty-six kilometres to the 

 sun's surface or through the suns atmosphere, has as much work done 

 on it by gravity, as corresponds to a high estimate of chemical 

 energy in the burning of combustible materials. But chemical 

 combinations and dissociations may, as urged by Lockyer, in his 

 book on the ' Chemistry of the Sun,' just now published, be 

 thoroughly potent determining influences on some of the features 

 of non-uniformity of the brightness in the grand phenomena 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 to which only allusion can be made at the 

 present time. 



What concerns us as to the explanation of sun-light and sun-heat 

 may be summarised in two propositions : — 



(1) Gigantic currents throughout the sun's liquid mass are con- 

 tinually maintained by fluid, slightly cooled by radiation falling 

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

 place. 



