652 Scientific Proceedings, Royal Dublin Society. 



" Lensfielb Cottage, Cambeidge, 

 " 26th February, 1887. 

 " My deae Sib, — 



" I am afraid you will think I have forgotten the question you 

 asked me in your letter. It is not so. But I thought that before 

 answering it I would look into Professor Suess's book, if I could find 

 it. It is not, however, either in the University Library or in that of 

 the Eoyal Society. But if I had found it I do not know that I should 

 have been much the wiser ; for, as you say, he gives no numerical 

 data as to the dimensions of the supposed continent, nor does he 

 specify what the continent actually is, if he is dealing with a real, not an 

 ideal, continent. 



"In a Paper 'On the Variation of Gravity at the Surface of the 

 Earth,' which I wrote long ago, and which is published in the 

 Transactions of the Cambridge Philosophical Society, 1 and in my collected 

 Papers, of which as yet only two volumes have appeared, I showed 

 that the effect of a continent would be to make a slight apparent 

 diminution of gravity in continental stations as compared with de- 

 tached oceanic islands. It operates in this way : — that the attraction 

 of the land causes the surface of the sea level, the level surface, that is, 

 which would be determined by a system of geodetic levelling carried 

 from the coast inwards, to stand higher from the centre of the earth 

 than it would have done had the place of the continent been occupied 

 by ocean. The raising of the sea-level is greatest inland ; but it is 

 quite sensible, and even important, at the coast itself of the continent. 



" How much the rising amounts to depends, of course, on the 

 dimensions you attribute to the continent, and the height you give it 

 above the undisturbed level of the sea. To take a numerical example, 

 I suppose the case of a circular island or continent, whichever you 

 please to call it, one thousand miles in diameter, and elevated a quarter 

 of a mile above the sea-level. I suppose the depth of the ocean, in 

 which this island is supposed to be placed, to be two miles. 

 I make the usual suppositions as to the average density of the 

 rocks, &c, in the neighbourhood of the earth's surface, and as to 

 the mean density of the earth, which is fairly well ascertained. I 

 find the elevation of the sea-level in the interior of the island, 

 or continent (Australia), a good way from the coast, to be about 



1 Vol. viii. (1849), pp. 672-695. 



