WHAT IS KNOWN OF THE EARTH 809 



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 ac- 

 tion 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 regu- 

 late 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 influ- 

 ence the animate and inanimate world. In whatever direction we 

 turn are to be found alternations of what may be termed terres- 

 trial 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 atmosjihere, 

 as well as the little understood variations in terrestrial magnet- 

 ism, are consequences of the same general causes. 



The remarkable force inherent in the globe, known as terres- 

 trial 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 es- 

 tablished that there are two magnetic poles, one in each hemi- 

 sphere, at which the needle would point vertically upward and 

 downward. Their position, which is not coincident with the geo- 

 graphical poles, is found to have varied according to some yet un- 

 known law. In the year 1878 the northern pole was in latitude 

 70° north, longitude 96° west, and the southern in latitude 73i° 

 south, longitude 147-2-° 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, foUomng an 

 unsymmetrical line, which in 1878 lay almost wholly to the north 



of the terrestrial equator in the hemisphere east of Greenwich, 



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