252 FROM NEBULA TO NEBULA 



these may easily have been due to the convulsive shak- 

 ings of the solar web. These would, as a matter of 

 course, precipitate earthquakes on a grand scale, frac- 

 turing the crust in a multitude of places; and through 

 the fissures thus created the imprisoned gases would 

 make haste to escape, thereby not only giving rise to 

 numerous volcanoes and much fouling of the atmosphere, 

 but incidentally letting down the previously distended 

 crust. Nor should we here neglect to take account of the 

 lateral stresses due to the gravitation of the land masses 

 toward the north pole, for it is just at such a time of 

 equilibristic readjustment as this that this lateral pres- 

 sure should make itself most felt and improve the oc- 

 casion by raising up the great mountain chains. 



The problem as to just how often these central ex- 

 plosions of the sun occur may find its answer in the re- 

 sponse geologists shall give to the question as to the 

 frequency of ice ages. If, in the absence of definite 

 knowledge, we assume the earth to be a billion years old 

 and the average interval between successive ice ages 100,- 

 000 years, there should have been, to date, 10,000 such 

 ages and 10,000 such cataclysmal explosions. It is very 

 doubtful, however, whether geology, on account of oblit- 

 erations of its records, can supply a surer answer to this 

 query than solar physics may more directly and speedily 

 yield. 



With the escape of the buoyant gases from within 

 him, the sun would naturally collapse upon himself, and 

 there would follow a long period of rejuvenation during 

 which his radiation, color and brilliancy would all be 

 those of a star of lesser mass; and during this same 

 period, which must be regarded as an abnormal and fit- 

 ful stage, he would, furthermore, exhibit fluctuations in 

 all the three attributes mentioned, making of him, for the 

 time being, a typical variable star. 



Although solar and stellar explosions are auto- 

 matically produced by the causes just outlined, it does 

 not follow that these are the sole agents. There must 

 always lie in the back-ground the possibility of collision, 

 not so much with another star, which is a very remote 



