38 On Local Attraction. [No. 1, 



involving three unknown quantities. The assumption which I then 

 make is, that the Mean Figure of the Earth is a spheroid ; that is, 

 that these three ellipses are all the same. The effect of this is to give 

 me four equations of condition, involving the three unknown 

 quantities. These I solve by the method of least squares. The result 

 is that the unknown deflections all come out very small ; and the 

 semiaxes of the three ellipses come out remarkably near each other in 

 value. The first part of this result shows, what I have intimated in 

 para. 2, that the local attraction arising from invisible causes hidden 

 in the solid crust of the earth must be such, as very nearly to 

 compensate for the effect produced by visible causes at the surface 

 existing in mountains and oceans. And the second part of the result 

 gives a very satisfactory solution of the problem of the Mean Figure 

 taking local attraction into account, making the semiaxes 



20926189 and 20855316 feet 



and the ellipticity — ^ 



5. In the fourth or last section of the Paper I enter into specu- 

 lations regarding the Constitution of the Earth's Crust, suggested by 

 the result of the preceding section. The following extract will best 

 represent my views on this interesting subject : — 



" The first thing I observe in the results given in the last paragraph is 

 the very small amount of the resultant deflections at the two extremities of 

 the Indian Arc — Punnce close to Cape Comorin, and Kaliana the nearest 

 station to the Himalaya Mountains ; whereas the effect of the Ocean and 

 the Mountains has been shown to be very large. This shows that the effect 

 of variations of density in the crust must be very great, in order to bring 

 about this near compensation. In fact the density of the crust beneath the 

 mountains must be less than that below the plains, and still less than that 

 below the ocean-bed. If solidification from the fluid state commenced at 

 the surface, the amount of contraction in the solid parts beneath the 

 mountain-region has been less than in the parts beneath the sea. In fact, 

 it is this unequal contraction which appears to have caused the hollows in 

 the external surface which have become the basins into which the waters 

 have flowed to form the ocean. As the waters flowed into the hollows thus 

 created, the pressure on the ocean-bed would be increased, and the crust, so 

 long as it was sufficiently thin to be influenced by hydrostatic principles of 

 floatation, would so adjust itself that the pressure on any couche de niveau 



