330 POPULAR LECTURES AND ADDRESSES. 



globe. But when we deal with real sea-water 

 we have a piece of very delicate and nice mathe- 

 matical book-keeping which reminds one of certain 

 rules in Compound Interest. The ice attracts the 

 fluid it displaces ; but the displaced fluid itself 

 attracts the remaining fluid and so contributes 

 to the resultant attractive force. First calculate 

 the amount of the attraction on the water due 

 to the ice-cap alone ; then calculate the in- 

 crease of the attractive force due to the displaced 

 water ; then calculate this increase, to the second 

 degree of approximation, and so on ; and thus you 

 get at the result. There have been different cal- 

 culations founded on largely varied assumptions 

 for data. Mr. H. D. Heath and the Rev. O. 

 Fisher made calculations which differed somewhat 

 widely from mine as well as from Croll's, but I 

 believe them all to be consistent. My result, 380 

 feet, seemed to be immensely smaller than the 

 others. In point of fact, Croll omitted to notice 

 that mine referred to an ice-cap gathering on an 

 "ideal set of islands, and that I supposed the whole 

 land to be distributed uniformly, and the ice to be 



