Local Attraction on Geodetic Operations. 



275 



to the theory of contraction of the crust, the parts near Dunkirk may 

 have been somewhat hollowed out by the scouring of the tide through the 

 Straits of Dover, so as to give the land, low as it is, every advantage in 

 deflecting the plumb-line south. I have no means of knowing the cha- 

 racter of the ground north of station (7) on the coast of Dorset. There is 

 no difficulty, however, iu accounting for the north deflection at that place, 

 and even for a greater deflection, if the attraction of the country north of 

 it is as much as the attraction of the land on Burleigh Moor on the north 

 coast of Yorkshire. To this station I shall revert. With regard to sta- 

 tions (9) and (10), I gather the following information from the Ordnance 

 Survey Volume. *'At present there are no sufficient data for calculating 

 exactly the disturbance" at Cowhythe (p. 662). It is supposed not to 

 exceed 6" (p. 664) ; but the calculation is not made for any part of the 

 mountains further south than 50 miles. The south deflection to be ac- 

 counted for, viz. —3"' 11, may in part be thus explained; or, even if, as 

 before, the North Sea is supposed to have been formed by the contraction 

 of the crust, the confined portion between the north coast of Aberdeen and 

 the Orkney Islands may have been formed by the removal of the superfi- 

 cial strata by currents so as to produce a deficiency of attracting matter. 

 So with respect to the other station, Ben Hutig, the unaccounted-for de- 

 flection, which is much smaller, viz. —0*60, maybe easily explained, as 

 the eff'ect of the land has not been calculated further off than about 3 miles 

 (pp. 660, 661). Thus, on the whole, the deflections at those coast-stations, 

 •where it is towards the land, can be pretty well accounted for, without call- 

 ing in aid the deficiency of attraction of water and supposing that the crust 

 below the ocean is not condensed. 



The seven coast-stations of the second list, where the deflection is towards 

 the sea, seem to bear individual testimony to the truth of the theory, that 

 the crust below the ocean must have undergone greater contraction than 

 other parts of the crust. The four stations (3), (4), (5), (6) on the south 

 coast of the Isle of Wight all have deflections southwards ; and their mag- 

 nitudes diminish in the order that the distances from the sea increase, — that 

 order being (3) High Port Cliff, (5) Boniface Down, (4) Week Down, (6) 

 Dunnose (see the Contour Map of Isle of Wight in the volume of Plates 

 accompanying the Ordnance Survey Volume). The amounts of the deflec- 

 tion seem almost to prove too much for the theory. Still they are all 

 in the direction of the ocean, and seem certainly to indicate that there is a 

 redundance of matter, and not a deficiency, in that direction. Blackdown 

 (7) is somewhat further inland than Dunnose is. If, then, the ocean and 

 crust together do really produce the outstanding deflection southward at 

 Dunnose, we shall have to suppose that the north deflection at Blackdown 

 in the first list of coast- stations, arising from the land, is not much less 

 than 2*76 + r73=4-49, which is a little less than the calculated deflection 

 at Burleigh Moor on the coast of Yorkshire, and is therefore not an unlikely 

 amount. The other three coast-stations, (8), (11), (13), all bear out the 



VOL. XIII. Y 



