Page:Popular Science Monthly Volume 33.djvu/829

Rh differences of climate, and corresponding variations in the forms and distribution of living creatures, vegetable and animal. Thus it is that while every part of the earth has its own characteristics, the general system of nature is one and the same everywhere; the special characters of the several regions being due to the action of local features or conditions, which are no sooner called into existence than they in turn become secondary efficient causes of the infinitely varied phenomena that our globe presents to us. In this manner has been evolved the face of nature as we now see it; nature which, working with never-varying forces, appears to man in the present as his type of stability, while it is constantly leading, through ever-varying forms, from the hidden shapes of an impenetrable past to those of an unknown future.

The influence of the movements and figure of the earth may everywhere be traced among the phenomena brought to our knowledge by the more and more complete exploration of its surface. The daily and annual motions of the globe, subject to the effects of the spherical form of the earth and the direction of its axis of rotation, determine at all parts of its surface the amount of heat and light received from the sun, and thus regulate all the conditions of existence upon it; they give rise to the varying length of days and of seasons at different places, and to a multitude of recurring phenomena which characterize or influence the animate and inanimate world. In whatever direction we turn are to be found alternations of what may be termed terrestrial work and rest, day and night, summer and winter, periodical winds extending over longer or shorter periods, seasons of rain and dry weather. The tides of the ocean, and the less apparent though not less regular periodical oscillations of the atmosphere, as well as the little understood variations in terrestrial magnetism, are consequences of the same general causes.

The remarkable force inherent in the globe, known as terrestrial magnetism, which gives a determinate direction to a freely suspended magnetic needle, and is of inestimable value to man, has long been the subject of observation and study. It is now established that there are two magnetic poles, one in each hemisphere, at which the needle would point vertically upward and downward. Their position, which is not coincident with the geographical poles, is found to have varied according to some yet unknown law. In the year 1878 the northern pole was in latitude 70° north, longitude 96° west, and the southern in latitude 73° south, longitude 147° east. Between these poles, a line that has been termed the magnetic equator, where the needle assumes a horizontal position, is found to pass round the earth, following an unsymmetrical line, which in 1878 lay almost wholly to the north of the terrestrial equator in the hemisphere east of Greenwich,