CHROMOSPHERE AND SOLAR PROMINENCES. 395 



each radial line represents the number of spots or protuberances ob- 

 served at each particular latitude on a scale of a quarter of an inch to 

 the hundred ; for example, Secchi gives 228 protuberances as the num- 

 ber observed during the period of his work between 10° and 20° of 

 south latitude, and the corresponding line drawn at 15° south, on the 

 left-hand side of the figure, is therefore made fff or .57 of an inch 

 long. The other lines are laid off in the same way, and thus the irregu- 

 lar curve drawn through their extremities represents to the eye the 

 relative frequency of these phenomena in the different solar latitudes. 

 The dotted line on the right-hand side represents in the same manner 

 and on the same scale the distribution of the larger protuberances, 

 having an altitude of more than 1' or 27,000 miles. 



A mere inspection of the diagram shows at once that, while the 

 prominences may, and in fact often do, have a close connection with 

 the spots, they are entirely independent phenomena. 



A careful study of the subject shows that they are much more close- 

 ly related to the faculse. In many cases at least, faculae, when followed 

 to the limb of the sun, have been found to be surrounded by promi- 

 nences, and there is reason to suppose that the fact is a general one. 

 The spots, on the other hand, when they reach the border of the sun's 

 image, are commonly surrounded by prominences more or less com- 

 pletely, but seldom overlaid by them. Indeed, Respighi asserts (and 

 the most careful observations we have been able to make confirm his 

 statement) that as a general rule the chromosphere is considerably de- 

 pressed immediately over a spot. Secchi, however, denies this. 



The protuberances differ greatly in magnitude. The average 

 depth of the chromosphere is not far from 10" or 12", or about 5,000 

 or 6,000 miles, and it is not, therefore, customary to note as a promi- 

 nence any cloud with an elevation of less than 15" or 20" — 7,000 to 

 9,000 miles. Of the 2,767 already quoted, 1,964 attained an altitude 

 of 40", or 18,000 miles, and it is worthy of notice that the smaller 

 ones are so few, only about one-third of the whole: 751, or nearly 

 one-fourth of the whole, reached a height of over 1', or 28,000 miles ; 

 the precise number which reached greater elevations is not mentioned, 

 but several exceeded 3', or 84,000 miles. There are numerous instances 

 on record, by different observers, of protuberances exceeding 100,000 

 miles, and a single instance, observed by the writer, in which the 

 enormous altitude of 7' 49", or 211,000 miles, was attained. 



In their form and structure the protuberances differ as widely as 

 in their magnitude. Two principal classes are recognized by all ob- 

 servers, the quiescent, cloud-formed, or hydrogenous, and the eruptive 

 or metallic. By Secchi these are each further subdivided into several 



nence near the pole would be carried but slowly out of sight by the sun's rotation, it is 

 thus easy to see how the number of prominences recorded in the polar regions is so large, 

 notwithstanding the smaller area of each zone of 5° width, as compared with a similar 

 zone near the equator. 



