Page:Popular Science Monthly Volume 66.djvu/516

512 definite connection between the number and size of the spots on the sun's surface and the weather or climate on the earth. On the other hand, theoretical reasons would lead us to expect some connection, and the evidence is rather in favor of it than otherwise; but it is highly probable that the direct effect of the spotted area is unimportant compared with the effects produced in our atmosphere by other causes.

Although a negative answer must at present be given to the question whether sunspots have any considerable share in the variations of climate, it would be leaving a wrong impression if no mention were made of a well-established 11-year period in the variations of another set of terrestrial phenomena. The state of the earth's magnetism has for many years been carefully recorded by suitable instruments and there is no doubt that its fluctuations correspond quite closely with the state of the sun's surface, not only in the cycle of long period, but also with respect to the minor variations. It can not as yet be stated that the appearance of a group of sunspots always causes a disturbance of the magnetic needle, but investigations have been published and others are still under way which appear to show a very intimate connection between them. Nothing is known as to the causes of this connection.

We have been dealing hitherto with only one exhibition of the solar activity. It is one which catches the eye and is perhaps well adapted to show the main features of this activity in somewhat the same way as the presence of snow and ice would indicate the higher peaks of a mountain range. Pursuing the simile, while it is true that we get an idea of the more prominent outlines of the mountainous region by noting the white places on the dark background, the latter would include many elevations and depressions which are never touched by snow, and we can not have a correct idea of the range without finding some method of observing them also. The case of sunspots is not very different. They probably form the bolder outlines, but, as Professor Bigelow has pointed out, they constitute but a sluggish register of the solar activity. There is another set of variable phenomena, known as the prominences, which are now supposed to furnish a much better index of the state of the sun at any time. These are elevations of the material of which the sun is composed, visible round the edge of the disc, and they appear to be huge masses of liquid or gaseous matter violently ejected from the main body. Unlike the sunspots, prominences are rarely absent and some of them can nearly always be seen on the edge of the sun's disc, so that by recording the number and height of those observable from day to day a much more continuous and accurate series showing the activity of the sun can be obtained. Moreover, it has been shown that the variations in the series correspond much better with those in the earth's magnetic force than