Connected with the Earth's Field of Force. 137 



these lines by two articles^* published quite independ- 

 ently of each other, in which the authors evolved the hy- 

 pothesis of continental creep in explanation of mountain 

 building. If we accept this idea, the inevitable question 

 is: what forces caused this motion of the continental 

 masses ?^^ 



Now the continents are certainly not huge spheres 

 rolling on the earth's surface. If we adopt the floating- 

 crust theory, they would be comparable to bodies floating 

 almost submerged in a liquid magma. If the floating 

 bodies were of almost the same density as the magma, 

 they would float almost exactly level with its surface and 

 could be considered as solidified portions of it, so that, 

 according to a well-known principle of hydrostatics, they 

 would be in equilibrium if the liquid containing them were. 

 But if the floating bodies were lighter and projected above 

 the level surface of the sustaining liquid, there would be 

 forces acting of just the same nature as those that draw 

 the sphere toward the equator.^ ^ The problem of the 

 sphere was given in some detail merely because it was 

 easier to reduce it to a concrete numerical example. The 

 forces acting are in a general way proportional to the 

 average elevation of the continental mass above sea level 

 and to the sine of twice the mean latitude of the continent, 

 and so would be greatest for a mean latitude of 45°. It 

 is not difficult, moreover, to make out, if we are dealing 

 with a floating continental mass longer than it is -wide 

 and lying with its greatest length neither along the meri- 

 dian nor perpendicular to it, that there will be twisting 

 forces called into play that would tend to set the axis of 

 greatest length at right angles to the meridian when the 

 mean latitude of the continent is less than 45° or that 

 would tend to set the axis of greatest length along the 

 meridian when the mean latitude is greater than 45°. 



"Taylor, Bulletin of the Geological Society of America, vol. 20, p. 625, 

 1910. Wegener, Petermann's Geographische Mitteilnngen, 1912, pp. 185, 

 253, 305. Much the same matter was later issued in book form under the 

 title: Die Entstehung der Kontinente und Ozeane (Brunswick, 1915). 



^" An attempt to see whether the , supposed shifting of continents con- 

 tinues and is large enough to be perceptible in the course of a few decades 

 was interrupted by the outbreak of the war. The plan was to redetermine 

 transatlantic differences of longitude and to compare the results with earlier 

 ones. 



^' For the mathematical developments see appendix B to this paper. 



