CHAPTER XXI. 



THE SUN. 



Reference has frequently been made, in the course of the 

 foregoing chapters, to the effects of solar heat upon the 

 earth. Such references, however, have been incidental 

 rather than direct ; and little, or nothing, has been said, of 

 set purpose, about the 'sun itself. It is proposed, therefore, 

 in this concluding chapter, to give a simple sketch of 

 what we know about the nature of the sun ; and to show 

 that the influence of this body may be regarded as the 

 prime mover in the production of most of the phenomena 

 which are exhibited within the basin of the Thames. 



When the sun is shining in its full splendour, it is much 

 too dazzling an object to be looked at with the unprotected 

 eye. Viewed, however, through a misty atmosphere, or 

 tlirough a dark-coloured glass, it presents the appearance of 

 a luminous disc, which is usually perfectly circular in shape 

 and homogeneous over its entire surface.^ The size of this 

 bright disc does not, however, remain precisely the same 

 throughout the year. It was explained in the last chapter 



^ " Usually," because the shape of the disc is sometimes distorted by 

 atmospheric refraction, while its uniformity is occasionally, though 

 rarely, broken by dark spots large enough to be seen by the unassisted 

 eye. 



