The Rev. W. Whewell on the mean Level of the Sea. 323 



to Mr. Thomas's remarks. Mr. Thomas thinks that the 

 sea is probably kept above its average height at Axmouth, 

 one of our stations, by the projection of Cape la Hogue into 

 the English Channel. But from what has been said, it must 

 be plain that this circumstance cannot alter the mean height 

 of the sea on either side of this supposed barrier. A chan- 

 nel fifty miles wide, between Cape la Hogue and the Bill of 

 Portland, is certainly enough to allow the water to assume its 

 mean level, as determined by the mean forces ; especially 

 when we consider that it has had an indefinite time allowed 

 to do this in. Indeed, we may satisfy ourselves that this con- 

 traction of the surface is no valid reason for the mean surface 

 being higher on the eastern side, if we recollect that it is just 

 as valid a reason for the surface being lower on that side ; 

 since the contraction prevents the water finding its level one 

 way as much as the other. 



It is true that a contraction of a channel in some cases 

 elevates high water; but this is precisely because high water is 

 a case of fluid motion, not of fluid equilibrium. We may ap- 

 ply to such cases the principle of the conservation of vis viva. 

 Each particle of fluid can temporarily ascend to the height 

 due to its velocity ; and hence an increased velocity, arising 

 from a contracted channel, makes an increased temporary 

 elevation possible. This consideration also explains the rapid 

 changes in the amount of tide, which take place as we pro- 

 ceed from one part of the sea to another. And hence we 

 see how little we are justified in drawing any inference con- 

 cerning levels, from any facts of high or low water. The 

 total tide on the coast of Wexford and Wicklow is not more 

 than three feet ; on the opposite coast of Wales, (the sea be- 

 ing very narrow,) it is between 20 and 30 feet. If the surface of 

 low water were a level surface, as Mr. Thomas supposes, we 

 should have the mean surface of one side of this narrow sea 

 constantly 10 feet higher than the mean surface of the other 

 side; a result quite impossible: whereas a temporary eleva- 

 tion or depression in one part of a fluid surface seven or eight 

 times as great as in another part, is quite familiar to us, and 

 offers no difficulty. 



In order to show that the mean surface of the sea is not 

 constant, Mr. Thomas refers to observations of his own, made 

 at Falmouth, Bristol, and other places. Upon these I would 

 beg to remark, that he appears to me to consider principally 

 detached observations ; whereas we cannot hope to arrive 

 at any satisfactory result, in any other way than by taking the 

 means of series of observations. We know well that special 

 causes, some of them known and some yet unreduced to rule, 

 perpetually affect particular high waters and low waters ; and 



Y2 



