Open main menu

Page:Popular Science Monthly Volume 72.djvu/497

This page has been validated.
493
THE EARTH AND THE SUN

presently show, seem to indicate an even closer coincidence between solar activity and the occurrence of earthquakes and volcanic eruptions.

Lockyer, Bigelow, Brückner, Clough and others have described climatic cycles averaging 3, 11, 36 and 300 years in length, all closely coincident with variations in solar activity. The 36-year cycle of Brückner is the best established and most easily recognizable. It is now accepted by the majority of meteorologists. It appears to pertain to the continental regions of the earth as a whole, although there are certain parts of the land close to the sea where the changes are the reverse of those in the interior. As to the seas data are not available. In continental regions the temperature is comparatively low at one extreme of the cycle; storms, clouds and precipitation are relatively abundant; storm-tracks of temperate regions approach the equator; snow lies long in winter; glaciers and rivers increase in size; lakes, especially those having no outlets, stand at a high level; and vegetation and animal life are appropriately influenced, as is evident from the time of the ripening of crops, and the expansion of irrigated areas in arid regions. These conditions prevail regularly at an interval of a few years after periods of exceptional activity in the sun. During the period of activity the 11-year sunspot cycle is reduced to 9 or 10 years, and there are other signs of unusual movement in the solar atmosphere. The other extreme of the Bruckner cycle follows a period of comparative inactivity in the sun, and is characterized by climatic phenomena the opposite of those just described. Meteorologists are not yet agreed as to the cause of the climatic cycles, but it seems to be well established that they are somehow connected with the sun.

In regard to the relation of solar activity to earthquakes and volcanoes, there is at present no agreement among students. On the whole, the evidence has seemed to most investigators to indicate that there is no relation. This appears to be largely due to the use of individual cases instead of averages, and to an attempt to find a coincidence between telluric activity, manifested in earthquakes and volcanoes, and maximum epochs of solar spottedness. Jensen, however, who has taken up the subject in a comprehensive fashion in volume thirty-six of the "Proceedings of the Royal Society of New South Wales," has come to a different conclusion. He has compiled a list of notable earthquakes and volcanic eruptions from 1783 to 1902. Each occurrence has been assigned a value of one, two, three or four, according to its severity, and all the earthquakes and eruptions for the whole series of years have been plotted as shown in Fig. 1. Having in this way obtained a graphic representation of the intensity of telluric activity in each year, Jensen added a curve showing the occurrence of sunspot maxima and minima. An inspection of the diagram thus obtained shows that