EEACTION OF PLANETS UPON SUN PUISEUX. 163 



than the disk, they have a special spectrum and during total eclipses 

 are the principal source of light. We can now photograph them at 

 any time about the edge of the disk by an ingenious method devised 

 in 1868 by Janssen and by Lockyer and since singularly perfected. 

 On many occasions we have been assured by incontestable evidence 

 that protuberances can mount in a few hours in the form of vertical 

 jets, narrow at the base to prodigious heights — 50,000 to 100,000 

 kilometers or even more. Generally, however, before attaining such 

 heights the protuberances expand into sheaves or stratified layers. 

 At times they seem to be the seat of violent explosions, are scattered, 

 and disappear very quickly. The spectroscope shows us that cal- 

 cium vapor, despite its atomic weight 40 times heavier than that of 

 hydrogen, rises very high in the protuberances. The displacements 

 of the spectrum lines also furnish confirmation of the enormous 

 velocities (100 kilometers or more per second) which the deforma- 

 tions of the contours suggest. 



Total eclipses, during which protuberances first attracted atten- 

 tion, are even now the only occasions when we can see another inter- 

 esting phase of solar activity — the solar corona. Sometimes it 

 apj)ears as a halo somewhat equally distributed around the disk, at 

 other times as gigantic streamers stretching out distances several 

 times the diameter of the sun. The forms of these rays indicate that 

 the matter of which they are composed shows no haste in falling 

 back into the sun. This matter is evidently very sparse and has very 

 little absorptive action on light, for, despite its irregular distribution, 

 it causes no difference in the appearance of the various parts of the 

 disk. Its mobility must be very great since in the interval of two 

 or three years between eclipses its structure completely changes, as 

 our photographs assure us. 



Spots, protuberances, and corona are subject to a great variation 

 which takes place regularly about nine times in a century. After a 

 period when the sun's disk appears entirely immaculate, spots re- 

 appear in both hemispheres at latitudes from 20° to 30°, then, always 

 increasing, they invade the equatorial regions, becoming at the max- 

 imum 20 times more numerous on the average than in a minimum 

 year. Then, as the decline commences, the numerical predominance, 

 which the Northern Hemisphere at first seemed to show, passes to the 

 Southern Hemisphere. The spots first disappear in the high lati- 

 tudes and then diminish all over the sun. 



The protuberances pass through a similar cycle, except that dur- 

 ing the period while their number increases their mean latitude tends 

 to increase in each hemisphere. Toward the epoch of spot maximum, 

 and only then, it is not rare to see great protuberances even near the 

 poles, where spots never appear. 



