Page:Popular Science Monthly Volume 4.djvu/410

394 the instrument at each point, to make the slit tangential to the limb.

The number of protuberances of considerable magnitude (exceeding 10,000 miles in altitude), visible at any one time on the circumference of the sun, is never very great, rarely reaching twenty-five or thirty; perhaps during the past few years it would most commonly lie between ten and twenty. At present, as the number of the spots decreases, the number of the prominences seems also to be diminishing, and within a few months there have been occasions when a careful search revealed only three or four.

Their distribution on the sun's surface is in some respects similar to that of the spots, but with important differences. The spots are confined within 40° of the sun's equator, being most numerous at a solar latitude of about 20° on each hemisphere. Now, the protuberances are most numerous precisely where the spots are most abundant, but they do not disappear at a latitude of 40°; they are found even at the poles, and from the latitude of 60° actually increase in number to a latitude of about 75°.



The annexed diagram, Fig. 5, represents the relative frequency of the protuberances and spots on the different portions of the solar surface. On the left side is given the result of Carrington's observation of 1,414 spots between 1853 and 1861, and on the right the result of Secchi's observations of 2,767 protuberances in 1871. The length of