THE 

 LONDON, EDINBURGH AND DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[THIRD SERIES.] 



NOVEMBER 1840. 



XLVI. On the mean Level of the Sea. By the Rev. W. 

 WHEWELL. F.R.S., Professor of Moral Philosophy in the 

 University of Cambridge*. 



TN the Philosophical Magazine for August last, page 134, 

 and the following, are some remarks by Mr. Richard 

 Thomas, on the Account of a Level Line, published in the 

 Transactions of the British Association for 1838. The level 

 line was carried from the Bristol Channel to the English 

 Channel by Mr. Bunt, under my direction: the conclusion 

 which I drew from the operation was, that the mean height 

 of the sea was at the same level at the two extremities of the 

 line; and this conclusion Mr. Thomas disputes. I beg to 

 offer a few observations on the subject, suggested t>y his re- 

 marks. 



In the first place, I will make one or two observations on 

 the theoretical view of the subject. It is certain (although 

 this view does not appear to be familiar to most of those 

 who turn their thoughts to the question) that according to 

 theory, the mean surface of the sea (in open water) is invari- 

 able in its height ; and that however different may be the 

 level of high water, or of low water at different places, the 

 line of mean water is a level line. This will appear from the 

 following considerations. 



The time of high water is different at different places, and 

 thus there are, at any given moment, unequal pressures opera- 

 ting upon different portions of the sea. It is high water at the 

 Land's-end when it is low water at Beachy Head ; and there- 

 fore the fluid pressures in different parts of the English Chan- 



* Communicated by the Author. 



Phil, Mag. S, 3, Vol. 17. No, 111. Nov. 1840. Y 



