v The Earth a Planet 77 



of the manifestations of solar energy. Down -rushes of 

 comparatively cool gases from the upper regions of the 

 Sun's atmosphere are believed to be the cause of black 

 marks which are often seen on the photosphere and termed 

 sun-spots^ although sometimes many thousand miles in 

 diameter. Though apparently black, compared with the 

 intense glow of the rest of the surface, sun-spots really shine 

 with a light brighter than that of the electric arc lamp. 

 Photographs of the Sun's disc are taken daily in some 

 observatories in order to preserve a record of the number 

 and movements of sun-spots, and in this way much inform- 

 ation has been obtained on the subject. It has been 

 observed that spots usually originate at some distance on 

 either side of the Sun's equator, and for a time they increase 

 in size ; then beginning to diminish they travel toward the 

 equator and gradually vanish, being succeeded by others, 

 which are smaller and fewer. Finally, after about twelve 

 years or so, the whole set fades away, and a new series of 

 larger size appear and go through the same changes. 

 Periods when sun-spots are at a maximum succeed each 

 other at intervals of about eleven years, and relations have 

 been traced between them and the influence of the Sun's 

 radiant energy on the Earth. During total eclipses a halo 

 of silvery light, sometimes circular, sometimes spreading 

 out like great wings, surrounds the Sun. It is called the 

 corona, and is probably composed of fine particles of dust 

 either thrown off by the Sun or being attracted toward it 

 and shining, in part at least, by reflected light. 



117. The Spectrum of Sunlight is a continuous band of 

 colour crossed by an immense number of .black lines (the 

 more conspicuous of which are named in Fig. 8, 63), 

 showing that the light from some glowing solid or liquid 

 has reached us after traversing an expanse of cooler vapour. 

 Every year more of the lines in this spectrum are identified, 

 and those which are produced in the Earth's atmosphere 

 are being distinguished from those due to the Sun's. The 

 lines produced by absorption of light in the Earth's atmo- 

 sphere are best recognised by comparing the spectrum of 

 the Sun low in the sky, when they are strongest, with that 



