Vol. 69.] ANNIVERSARY ADDRESS OF THE PRESIDENT. lxxxi 



rate of the pendulum, both due to variations in gravity, have been 

 and are being made in various parts of the world, notably in Austria, 

 Germany, America, and India. In India it has been stated that 

 gravity is greater on islands than on the coast of the mainland, 

 greater on the coast than inland, and greater on the plains than in 

 the immediate neighbourhood of the Himalayas. Col. Burrard, 

 by recent observations with the plumb-line, has shown that a line 

 of high density crosses India in lat. 23° ST., and that between this 

 and the Himalayas exists a line of low density, or, as he terms it, a 

 ' deep rift,' in the approach to which the change in the amount of 

 deflection is remarkably rapid. The effects of the rift are so great 

 as to obliterate the effects of topography and isostasy. So far as 

 regards the immense alluvial plains of Northern India the plumb- 

 line is deflected everywhere away from the mountains, and the 

 deflections are in opposition both to the topography and the theory 

 of isostasy. 



The observations in Great Britain were carried so far as to 

 indicate the existence of variations in gravity which were not to be 

 explained by the configuration of the ground. Capt. A. B. Clarke, 

 writing in 1858 on discrepancies in latitude when geodetically 

 compared, concludes that 



' it must be assumed that every latitude is affected by two distinct sources of 

 disturbance : namely, superincumbent irregularly-disposed masses, and irregu- 

 larities in the distribution of matter below the surface.' l 



In the Isle of Wight he notices a southward deflection, whereas, 

 from the mass of England to the north a northward deflection 

 might have been anticipated (op. cit. p. 712) : an interesting ob- 

 servation when taken in connexion with what ia now known of 

 the deflection on coast-lines. At Lough Foyle, Ben Hutig, and 

 Edinburgh he found reason to infer ' that the plumb-line is drawn 

 southward by dense subterranean masses.' At Blackdown in 

 Dorset, Southampton, Greenwich, and in the Shetland Islands 

 deflections were noticed which could not be accounted for by 

 inequalities of the ground. At Portsoy, on the northern coast of 

 Banffshire, there is a local but remarkably large disturbance of 

 gravity which has not been explained. 



The famous experiments on Schiehallion in 1772, and on Arthur's 

 Seat in 1855, were made with the view of ascertaining to what 



1 ' Account of the Observations & Calculations of the Principal Triangu- 

 ktion, &o.' 1858, p. 706. 



VOL. LXIX. / 



