each radial line represents the number of spots or protuberances observed at each particular latitude on a scale of a quarter of an inch to the hundred; for example, Secchi gives 228 protuberances as the number 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 400 or .57 of an inch long. The other lines are laid off in the same way, and thus the irregular 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 closely related to the faculæ. In many cases at least, faculæ, when followed to the limb of the sun, have been found to be surrounded by prominences, 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 completely, 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 depressed 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 prominence 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 observers, 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.