THE PLAN OF THE EARTH AND ITS CAUSES. 381 



ing is more improbable than variations along- the axis of rotation, let us 

 consider whether there is any geodetic proof of these flattened faces 

 and projecting edges. 



There has been a long controversy as to whether Bessel's or Clark's 

 ellipsoid better represents the figure of the earth. Clark's figure was 

 the later in date, and is generally considered as the more exact. 

 Helmert therefore expresses some surprise that the gravitational 

 observations in central Europe along the fifty-second parallel of north 

 latitude agree with Bessel's curve better than they do with Clark's; 

 this is the case all across the area on which Bessel's own work was 

 done. But as soon as we get into the Volga Basin, the gravity line 

 diverges from Bessel's curve and approaches that of Clark. The 

 change comes due north of the Eurafrican meridional edge. The 

 anomalies are at once removed if we assume that both ellipsoids are 

 locally correct; that Bessel's curve is true for Europe, and Clark's 

 correct for Asia; and that the two merge into one another north of the 

 line of the Eurafrican tetrahedral edge. 



On the tetrahedral theory there ought to be a projection north of this 

 tetrahedral edge. And gravity determinations show a great deficiency 

 in gravity in western Russia in an appropriate area along the Volga 

 Basin. It is true that the figures have been queried. There is a natural 

 tendency to query all facts that do not agree with theory, and the 

 notes of interrogation in this case may illustrate that tendency. But 

 on the view that there is an upward deformation of the earth in this 

 area, the anomalous deficiency in gravity observations is at once 

 explained. 



It may be replied that the existence of a normal gravity attraction 

 at Moscow negatives the assumption of a superficial deformation, but 

 the relative excess of attraction there is possibly due to the outcrop of 

 Palaeozoic rocks, of greater density than the loose sediments of the 

 Russian lowlands. 



Passing from Russia to the area in North America, where the next 

 tetrahedral corner should occur, there is another area of deficient gravity, 

 which may also be due to that area being a tetrahedral elevation. The 

 deficiency is explained by the assumption of vast subterranean blocks 

 of very light material. But that explanation is prohibited in the Rus- 

 sian case, since, as Helmert has shown, the deviations of a plumb line 

 from the vertical are inconsistent with the existence of such blocks. 

 Iu reference to the North American case Helmert has remarked that 

 the light subterranean blocks must descend for several kilometers; and 

 Mendenhall has shown that no reasonable assumption will suffice to 

 explain the facts. 



It would be too much to claim that geodetical evidence at present 

 available proves the tetrahedal theory, for accurate data are not yet 

 available for a sufficient proportion of the earth to show whether the 

 major deviations are based on a regular plan ; but papers, such as that 



