302 Mr. J. Croll on the Physical Cause of the Submergence 



analogous phenomena. The rise of the ocean under the attrac- 

 tion of the ice-cap, as I have already shown*, is a purely statical 

 effect, whereas the tidal wave is as much a kinetic effect as a 

 statical. 



He states that the principle on which I have attempted to 

 solve the problem is altogether erroneous. I had expressed the 

 same opinion myself regarding one part of my principle, but in 

 rather too unqualified terms f. From the letter of Professor W. 

 Thomson appended, it will be seen that I had been led to abandon 

 the fundamental plan of calculation used in my original theory 

 somewhat too hastily. 



All those who have discussed the subject, with the exception 

 of Archdeacon Pratt, have assumed the earth to be a perfect 

 sphere previous to the-ice cap being placed upon it. I shall, 

 however, assume it to be a sphere after the addition of the ice- 

 cap. This assumption is allowable; for in reality an ice-cap of 

 the form which 1 shall assume actually makes the earth more 

 spherical than before, for it tends to make the polar diameter 

 more nearly equal to the equatorial. In regard to the form of 

 the cap, each one has chosen that form which appeared to him 

 best adapted to simplify calculations; and of course we are at per- 

 fect liberty to do the same. M. Adhemar, who was the first to 

 direct his attention to the subject, and Mr. Heath, and also 

 Mr. Fisher, assume the cap to be of uniform thickness. I shall, 

 however, assume it to be thickest at the pole, and to thin away 

 towards the equator. 



We shall begin with a period of glaciation on the southern 

 hemisphere. Let WNES' 



be the solid part of the earth, Fig. 1. 



and c its centre of gravity. 

 And let ESW be an ice-cap 

 covering the southern hemi- 

 sphere. Let us in the first 

 case assume the earth to be 

 of the same density as the 

 cap. The earth with its cap 

 forms now a perfect sphere 

 with its centre of gravity at 

 o; for WNES is a circle, 

 and o is its centre. Sup- 

 pose now the whole to be 

 covered with an ocean a few 



miles deep. The ocean will assume the spherical form, and will 

 be of uniform depth. Let the southern winter solstice begin 



* Reader, March 3, 1866. f Ibid. 



