Mr. D. D. Heath on the Problem of Sea-levels. 35 



nitude, generally speaking, as that of the ellipticity of the sur- 

 face. Taking my former illustration, the yelk of the egg will lie 

 on the big-end side of its centre of gravity. 



In applying this theory to facts, we must conceive our conti- 

 nents and islands as so many excrescences from the spherical 

 nucleus which will shift their positions along with it; so that the 

 change of level, as measured on a cliff or mountain, will be that 

 calculated in my first paper, due to the change of contour, in- 

 creased in one hemisphere and diminished in the other by an 

 amount proportional to the sine of the latitude, the change at 

 the equator being due to the former cause only. To complete 

 the theory, each of these excrescences must be treated as exerting 

 a disturbing force of the same kind as the supposed ice-cap, and 

 all the effects must be added togther. 



In the same April Number there was a paper by Mr. Croll, in 

 which he criticised my uncorrected paper, and also developed his 

 own theory; and he appended a note by Professor W. Thomson, 

 expressing a marked approval and acceptance of this theory, as 

 including all that is relevant to " the great physical question." 



As to the first part of Mr. CrolFs paper, 1 have only to say 

 that, were 1 rewriting my own, I should leave out my illustration 

 of his axiom by reference to the moon, but should retain that by 

 the sun. But if, as I imagine, he thinks that, were the moon 

 stayed in her motion and rigidly connected with the earth, the 

 water would tend to gather spherically round the common centre 

 of gravity, he is wrong. The sea would keep the elliptic shape 

 assigned to it by the common statical theory of the tides, but the 

 nucleus would be removed some 85 feet from the centre of the 

 ellipse*; whereas the common centre is 3000 miles off. 



As to Mr. CrolFs own theory, perhaps the fault is mine, but 

 certainly I and Professor Thomson read it differently. 



For Professor Thomson concludes with the very just remark — 

 obvious I should have thought it, but that Archdeacon Pratt 

 seems to include floating ice in his disturbing force (p. 174) — 

 " that a transference of floating ice goes for nothing ; and that in 

 estimating the effect of (/rounded icebergs, the excess of the mass 

 of ice above that of the water displaced by it is to be reckoned 

 just as if so much ice were laid on the top of an island." Now 

 on looking more closely into Mr. CrolPs theory (which I had 

 only glanced at in the c Reader } ), I see that his ice-cap is sup- 

 posed to be fitted to a lens-shaped nucleus at the bottom of the 

 sea with a mile or two of water flowing over it ! (p. 302 at bot- 

 tom, and both the figures). 



mass of moon excess of mass hi nucleus 

 * In ray notation, 5 : a : : (distance)2 : ~, 



D2 



