532 On the Level of the Sea during the Glacial Epoch. 



in connexion with the vibrations of which the lines are probably 

 produced ? The speculation presents itself, whether the occur- 

 rence of this one line only in the nebulae may not indicate a form 

 of matter more elementary than nitrogen, and which our analysis 

 has not yet enabled us to detect*. 



Observations on other nebulse, which I hope to make, may 

 throw light upon these and other considerations connected with 

 these wonderful objects. 



LXXVI. On the Level of the Sea during the Glacial Epoch. 

 By Archdeacon Pratt. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



SINCE I sent you a paper on the above subject, which was 

 published in your Number for March, I have seen several 

 letters which Mr. Croll's first letter has drawn forth ; and I per- 

 ceive that I have omitted to consider the effect of the drain upon 

 the ocean which the formation of Mr. Croll's ice-sheet would 

 cause. My calculation of the amount of rise in the level of the 

 ocean in passing from the equator to northern latitudes may be 

 all right. But the drain upon the ocean would reduce the level 

 so as to make my starting-point (the equator) much lower, I 

 conceive (probably 2000 feet) , than the rise caused by the attrac- 

 tion of the ice- sheet. This therefore most materially affects the 

 result. 



This demand for water to form the ice-sheet would be supplied 

 in part (but not sufficiently) from the southern hemisphere ; for 

 the attraction of the ice-sheet in the north would produce a de- 

 pression in the ocean at the south pole and parts thereabout, 

 and by no means an elevation. 



I say " by no means," in reference to the way in which some 

 of the writers and calculators in this problem have supposed that 

 they see an analogy between the perturbation of the ocean by the 

 moon and by the ice-sheet. But the cases are quite different. 

 The moon is a free body, and as it draws the ocean so it draws 

 the earth's centre ; and the difference of effects must be taken to 

 find the state of the ocean with reference to the earth's centre. 



* On January 9, 1866, the author observed the spectrum of comet I. 

 1866. The comet appeared in his telescope as an oval nebulous mass sur- 

 rounding a small and dim nucleus. The prism showed that the nucleus 

 was self-luminous, that it consisted of matter in the state of ignited gas, 

 and that this matter is similar in constitution to the gaseous material of 

 some of the nebula. The coma shines by reflected solar light. — Proceedings 

 of the Royal Society, vol. xv. p. o. 



