42 PHYSIOGRAPHIC GEOLOGY. 



Recapitulation. — From this survey of the continents and oceans it 

 follows : — 



That, while there are many variations in the courses of tlie earth's 

 feature-lines, there are two directions of prevalent trends, — the northwest- 

 erly and the northeasterly ; that the Pacific and Atlantic have thereby their 

 positions and forms, the islands of the oceans their systematic groupings, 

 the continents their triangular and rectangular outlines, and the very physi- 

 ognomy of the globe an accordance with some comprehensive law. 



It has been observed, first by Professor R. Owen, of Indiana (1857), that the outlines 

 of the continents lie in the direction of great circles of the sphere, which great circles are, 

 in general, tangential to the arctic or antarctic circle. By placing the north pole of a 

 globe at the elevation 23° 28' (equal to the distance of the arctic circle from the pole or 

 the tropical from the equator), and revolving the globe eastward, part of these conti- 

 nental outlines, on coming down to the horizon of the globe, will be found to coincide with 

 it ; and on revolving it westward, most of the other lines. Other great lines, as part of 

 those of the Pacific, are pointed out as tangents to the tropical circles instead of the arctic. 

 But there are other equally important lines which accord with neither of these two systems, 

 and a diversity of exceptions when we compare the lines over the surfaces of the conti- 

 nents and oceans. 



If the views of Mr. Owen are right, the direction of coast lines on the parallel of QQ° 32' 

 north or south should be east and west (being tangent to the antarctic circle), and on the 

 equator, about N. 23° 28' E. ; and the trend in other places intermediate between these 

 extremes. And in the tropical part of the ocean, great circles tangent to the tropical circles 

 would have the course N. 66° 32' W. crossing the equator, but be E.-W. on the tropical 

 circles ; and between the two positions between N. 66° 32' W. and E.-W. The map 

 Csee page 47) shows how far these are the actual courses. 



IV. OCEANIC AND ATMOSPHERIC MOVEMENTS AND 



TEMPERATURE. 



The earth has east-west differences, as already pointed out, in the depths 

 of its oceans (page 19). But of greater importance are the east-west or 

 front-and-rear contrasts which are a consequence of its diurnal revolution. 

 Ordinary observation recognizes only the rising and setting of the sun, and 

 day and night, as the chief consequences of the eastward rotation. But 

 these are only the most obvious. The results are manifested universally in 

 the climates of the globe, the winds, the tides, and oceanic currents, in the 

 earth's magnetic currents, in the geological action of waters of the ocean 

 and land, the distribution of plants and animals ; and they give to the 

 eastern and western sides of the continents, or the front and rear, differences 

 which are profound in influence both physically and physiologically. The 

 effects are diversified and extreme. They are moderated by the nutation of 

 the earth's axis in its annual revolution, which gives the earth its winters 

 and summers; but they are none the less real and fundamental. Ferrel 

 obtained a mathematical expression for the relation of the rotation to the 

 winds, and announced the fundamental law (1858) that " in whatever direc- 

 tion a body moves on the earth's surface, there is a force, arising from the 



