286 FROM NEBULA TO NEBULA 



of course, consists in discovering which is the most re- 

 fractory element or group of elements most conspicu- 

 ously represented in the pertinent spectra; and a good 

 deal also depends upon the age of the spot at the time 

 the spectra are obtained. 



Some may infer that I mean that these deep-seated 

 blasts occur sporadically, and without any preliminary 

 preparation of the surface. I do not mean this especi- 

 ally, though I admit the possibility of such happenings. 

 The time being ripe for the coming event, I presuppose, 

 rather, a series of eruptions, from the top layer to the 

 next, and so on, opening a path of least resistance down 

 to the lowest layer affected in each individual case. 

 With each succeeding deeper detonation the original 

 superficial vent is widened more and more until there 

 yawns at last a well thousands of miles in depth, ripped 

 through from below. This remains open until the inter- 

 nal pressure is sufficiently relieved, and then ensues a 

 falling and filling in until the gaping wound is complete- 

 ly healed. That the girth of the sun should diminish 

 slightly as a result of these belchings is self-evident, but 

 the difference is too small to have been measured, though 

 Professor Poor, it seems, has detected some such differ- 

 ence. 



Owing to the differential rotations within the solar 

 body, as previously described, and the friction caused 

 thereby, the pent up gases are electrified before their es- 

 cape, and, after escape, rising hundreds of thousands of 

 miles above the sun's surface, they produce and perenni- 

 ally keep in repair the coronal veil that envelopes him. 

 " There is a cycle of changes ", says Doctor Abbot, "sup- 

 posed to be identical with that of sun-spot frequency. 

 As the corona can be observed only at total solar 

 eclipses, the march of the cycle of changes is as yet only 

 imperfectly known, but for the last half century it has 

 been observed that there are long equatorial coronal 

 streamers at the time of sun-spot minimum while at maxi- 

 mum of sun-spots the corona extends only to moderate 

 distances but nearly uniformly in all directions from the 



