POPULAR SCIENCE 283 



a definite progression of changes. At first a few small spots 

 appear in bright compact faculae, or luminous white clouds, 

 the faculae preceding the appearance of the spots. These spots 

 then coalesce into two main spots, a leader or western spot of 

 the group, with a trailer or eastern spot, with many subsidiary 

 spots bridging the gulf between the two main spots of the 

 group. It is in this complex region between the spots that, 

 in the earlier days of the life-history of a group, the greatest 

 display of energy takes place. In this region spectroscopic ob- 

 servations show many reversals and distortions in the hydrogen 

 lines. Meanwhile the faculae are still very bright and cling to 

 the spots. The subsidiary spots bridging the gulf between the 

 leader and tail spot will first disappear. The tail or following 

 spot will next break up, leaving the leader spot to attain the 

 comfortable rotundity of middle age. Finally, this spot, 

 growing ever smaller and smaller, will disappear as a few 

 insignificant dots. The faculae, which in the early days of the 

 life of the spots clustered round the group in dense luminous 

 clouds, will extend in branching lanes to cover an immense 

 area of the solar surface. 



From a detailed study of some 3,500 drawings of solar spots 

 made at Stonyhurst, it was possible to divide all spots into 

 five normal types. But the chief type, of which the others are 

 but phases, was found to be the double spot formation. These 

 groups have subsequently received from Prof. Hale the name 

 of bipolar groups, and they exhibit opposite magnetic polarity. 

 In the northern hemisphere and in middle solar latitudes, 

 the direction of rotation for the preceding spot or western 

 spot is counter-clockwise, and for the eastern or following 

 spot clockwise. This direction of rotation corresponds in the 

 preceding spot to that of a terrestrial tornado. The opposite 

 direction obtains in the southern hemisphere, as is the case 

 also in terrestrial tornadoes. But in spots in high latitude 

 these directions are reversed. 



Sun-spots appear to be dark, the umbrae and particularly 

 the nuclei seeming to be very black, but in reality they are 

 intensely bright — far brighter than any terrestrial source of 

 illumination. They are composed of uprushes of metallic 

 vapours, in which vanadium, titanium, and iron are very 

 evident. The rapid expansion and consequent cooling rela- 

 tively to the bright solar surface cause them to absorb the 



