268 (J. II. I )<tf tn'u, - St ,'• xx< X rtlli, .svv/ /// ///r Eili'th hi/ 



We have concluded above that Africa and America are 

 about equivalent to an harmonic ot the fourth order, hence it 

 may be concluded that the stress-difference under those conti- 

 nents is a maximum at more than 1100 miles from the earth's 

 surface, and there amounts to about four tons per square inch. 

 A comparison with Table VII shows that marble would break 

 under this stress, hut that strong granite would stand. 



The case of the isolated continent investigated in section 

 eight appeared likely to prove the most interesting one, for the 

 purpose of application to the case of the earth. But unfor- 

 tunately I have found it difficult to arrive at a satisfactory 

 conclusion as to the proper height to attribute to the continent. 



The average height of the American continent is about 1100 

 feet above the sea, and the average depth of the Pacific Ocean 

 about 15,000 feet. If the water of the Pacific be congealed 

 into rock it will have an effective depth of 10,000 feet. The 

 greatest height of the American continent above the bed of the 

 dried Pacific when smoothed down must be fully 12,000 feet, 

 or 3700 meters. The height of the great central Asian pla- 

 teau above the average bed of the southern ocean (after drying) 

 must be coi ban this. 



Now, in the application to the homogeneous planet the 

 heights are to be halved to allow for the smaller density of 

 surface rock. I therefore take 2,000 meters as the height of 

 the top of the equatorial table-land ahove the remaining 

 approximately spherical portion of the sphere. 



The investigation of § 8 then shows that the equatorial 

 tahle-latid will give rise to a stress-difference of 4"1 tons 

 per square inch at a depth of 660 miles; and that the equato- 

 rial table-land counter!.;, lanced by the pair of polar continents 

 (the second harmonic constituent being absent), gives a stress- 

 difference of about 3*8 tons per square inch at a depth of 590 

 miles. 



This estimate of stress-difference agrees in amount, with sin- 

 gular exactness, with that just found from the case of the 

 'fourth zonal harmonic, but the maximum is reached 400 or 500 

 miles nearer to the earth's surface. 



I think there can be no doubt but that there are terrestrial 

 inequalities of much greater breadth than that of my isolated 

 continent; thus this investigation for the isolated continent 

 will give a position for the maximum stress-difference too near 

 the surface to correspond with the largest continents. On the 

 other hand, I do not feel at all sure that I have not consid- 

 erably umh !,t of such a comparatively nar- 



In the present paper it has been impossible to take any 

 notice of the stresses produced by the most fundamental 



