224 On the Constitution of the Crust of the Earth. [Dec. 22, 



thesis regarding the Constitution of the Earth's Crust, which he pro- 

 pounded in 1864, viz. : that the variety we see in the elevation and depression 

 of the earth's surface in mountains and plains and ocean- beds has arisen 

 from the mass having contracted unequally in becoming solid from a fluid 

 state ; and that below the sea-level, under mountains and plains, there is 

 a deficiency of matter approximately equal in amount to the mass above 

 the sea-level ; and that below ocean-beds there is an ;excess of matter ap- 

 proximately equal to the deficiency in the ocean when compared with rock ; 

 so that the amount of matter in any vertical column drawn from the sur- 

 face to a level surface below the crust is now, and ever has been, approxi- 

 mately the same in every part of the earth. 



In order to make this hypothesis the subject of calculation, the author 

 takes the case of the attenuation of matter in the crust below mountains 

 and plains, and the excess of matter below ocean-beds, to be uniform to 

 a depth m times the height above the sea-level or the depth of the 

 ocean, as the case may be. 



The results are shown in the following Table, in which the numbers are 

 the last figures in the ratio of the differences of gravity to gravity itself, 

 carried to seven places of decimals. The decimal point and ciphers are 

 omitted for convenience. 



Stations. 



Differences of gravity. 



Relative 

 effects of local 



attraction 

 deduced from 



pendulum- 

 observations. 



Residual errors after correction by the 

 method of 



Dr. Young. 



This hypothesis. 



m = 50. 



7»=109. 



Indian arc stations. 



+384 

 -323 

 +341 

 -707 









Coast stations. 



-562 

 -926 

 -208 

 -957 



- 78 

 -455 

 +338 

 + 69 



-557 

 -584 

 +315 

 +320 



Ocean station. 



+302 

 -160 

 -197 

 +142 



+894 



+314 

 -154 

 -192 

 +153 



+906 



+331 

 -122 

 -138 

 +216 



+ 31 



+ 360 



- 79 



- 78 

 +291 



+102 



The author points out from this Table that Dr. Young's, or the usual 

 method of correction for local attraction, so far from improving matters, 

 introduces very large residual errors of the arc and ocean stations ; and, at 

 places on the arc of meridian, all lying on the same side with reference to 

 Punnse. He observes that neither the usual method nor his own much 



