48 



THE TIDAL PROBLEM. 



of the caps increased at all points. The extension should have been great- 

 est at the poles and should have died away to zero at the parallel of no 

 chano-e. There was therefore a climacteric stretching at the poles and a 

 climacteric compression at the equator. 



Fig. 6 is intended to illustrate the nature of the change as seen from 

 a point of view above one of the poles. To keep the view as true to per- 

 spective as practicable, the equatorial belt is foreshortened. The excess 

 of area in the equatorial belt is represented by the black triangles, which 

 are too small on account of this foreshortening. The deficiency of area in 

 the polar region is represented by the white ground, which is more nearly 

 in true proportion. 



Changes in the crust of the earth of this magnitude, or of such lesser 

 magnitude as would have followed a change from any of the other early 

 periods of rotation to the present one, could scarcely have taken place 



without leaving a record of them- 

 selves in the form of compressional 

 and tensional phenomena. We 

 may, to be sure, suppose that the 

 interior of the earth has always 

 been sufficiently mobile, in one 

 form or another, to permit inter- 

 nal shift of material from areas of 

 compression to areas of tension, 

 and so to accommodate itself to the 

 progressive change of form, but 

 this can not reasonably be sup- 

 posed to have taken place in the 

 outer shell without having left evi- 

 dences of itself, for this shell must 

 be assumed to have been solid from 

 an early state and, being at the 

 surface, it was not under such pres- 

 sure as to fllow and hence must 

 have been deformed in the familiar 

 modes that characterize surface 

 thrust and tension respectively. It 

 is known from abundant geological 

 observation how the shell of the earth deports itself under conditions of 

 compression and tension resulting from forces of the kind that would arise 

 from the changes assigned. The data of SUchter's computations may there- 

 fore be interpreted by the usual methods. 



The equatorial belt of the earth of the 3.82-hour rotation-period would 

 differ from that of the present earth to the extent of a broad swell 180 

 miles high. In settling down this might doubtless relieve its excess of 

 length in cross section by thrusting northward and southward into the 

 areas of tension, but as its equatorial length was 1,131 miles greater than 

 the present equator, it would seem that in an east-west direction the tract 

 must fold, crumple, and overthrust on itself after the familiar fashion of 



Fio. 6. — Polar projection of earth's hemisphere 

 showing theoretical high-latitude tension and low- 

 latitude oompression involved in a change of rotation 

 from 3.82 houra to present rate. Figure is drawn to 

 true scale as seen from a point above pole, and in 

 consequence the equatorial tract is foreshortened. 

 The black triangles reduced in length by foreshorten- 

 ing show compression ; the white show tension in essen- 

 tially true proportions. The neutral line between areas 

 of compression and of stretching lies at 33° 20' latitude. 



