373 



Archdeacon Pratt on the effect of 



is taken, and estimated at + 19"*71 and +6"' 18, the effect of the mountain 

 mass on the north being +3"-00 and 27"-98. 



16. 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 extremi- 

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

 nearest station to the Himmalaya 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 influ- 

 enced by hydrostatic principles of floatation, would so adjust itself that the 

 pressure on any couche de niveau of the fluid shcitld remain the same. 

 At the time that the crust first became sufficiently thick to resist fracture 

 under the strain produced by a change in its density — that is, when it first 

 ceased to depend for the elevation or depression of its several parts upon 

 the principles of floatation, the total amount of matter in any vertical 

 prism, drawn down into the fluid below to a given distance from the 

 earth's centre, had been the same through all the previous changes. After 

 this, any further contraction or any expansion in the solid crust would not 

 alter the amount of matter in the vertical prism, except where there was 

 an ocean ; in the case of greater contraction under an ocean than elsewhere, 

 the ocean would become deeper and the amount of matter greater, and in 

 case of a less contraction or of an expansion of the crust under an ocean, 

 the ocean would become shallower, or the amount of matter in the vertical 

 prism less than before. It is not likely that expansion and contraction in 

 the solid crust would affect the arrangement of matter in any other way. 

 That changes of level do take place, by the rising and sinking of the sur- 

 face, is a well-established fact, which rather favours these theoretical con- 

 siderations. But they receive, I think, great support from the other fact, 

 that the large effect of the ocean at Punnce and of the mountains at Ka- 

 liana almost entirely disappear from the resultant deflections brought out 

 by the calculations. The formulae of paragraph 15 show that when we 

 get close to the mountain-mass, as at Dehra, which is at the foot of the 

 mountains where they first rise rapidly above the plains, the resultant deflec- 

 tion is very great ; the less density of the crust down below the sea-level 

 drawn under the mountain-mass has here a very trifling influence. This 

 is as it should be, if the depth of this less density is considerable; 



