THE HEAT OF THE SUN. 9 



chiefly, if not wholly, from the surface of the heated 

 body, and the heat from those parts which are not on the 

 surface can only be dispersed by radiation after it has 

 travelled by conduction from the interior to the exterior. 

 Such at least would be the case if the body were a solid 

 one ; if, however, it were either wholly or partly in a 

 liquid or in a gaseous condition, as the sun appears to be, 

 then the mode by which the heat would pass from the 

 interior to the exterior would be correspondingly modified. 

 There would, doubtless, be currents of convection in the 

 solar materials just as there are currents of convection 

 which distribute, throughout the bulk of the liquid, the 

 water that has been heated at the bottom of a kettle 

 placed on the fire. This does not contradict the statement 

 that we have made as to the necessity for the arrival of 

 the heat from the interior at the surface before it could be 

 dispersed by radiation. The mode of conveyance of the 

 heat must be different in a fluid from what it is in a solid, 

 but the general principle remains unaltered. The spots 

 on the sun are doubtless connected with the circulation of 

 the heat between the interior and the surface. The 

 character of these remarkable features on our great lumi- 

 nary is well shown by the reproductions of the Rev. F. 

 Hewlett's photographs of spots in 1882, 1883. 



The extraordinary profusion with which heat is poured 

 forth from every square foot of the sun's surface may 

 perhaps be explained by the following illustration. Sup- 

 pose there are two concert-halls, built from like designs, 

 but with every dimension in one of the buildings double 

 that corresponding in the other. The area, for instance, 

 in one hall is twice as long and twice as wide as in the 



