324 The Rev. W. Whewell on the mean Level of the Sea, 



therefore affect the mean waters as deduced from these. Thus, 

 for instance, the diurnal inequality at Bristol might make two 

 successive high waters differ by four feet; and thus would 

 make the height of mean water differ by two feet, according 

 as the one or the other of these high waters was compared 

 with the intermediate low water. Add to such inequalities 

 those arising from the varying pressure of the atmosphere, 

 the wind, and the like, and we may easily conceive the pos- 

 sibility of such deviations from constancy in the half-tide 

 mark as Mr. Thomas records. Yet these deviations do 

 not prevent the mean of any fortnight's observations from 

 being constant within a very few inches ; as it will be found 

 to be at Liverpool, Plymouth, and any other place similarly 

 circumstanced. 



It will be observed, that I allow that in a river the line of 

 mean water slopes towards the sea ; but this condition does 

 not affect either end of our level line ; and therefore I consi- 

 der that the result which we obtained, namely, that the height 

 of mean water is the same at its two extremities, represents 

 the general condition of the ocean. 



Mr. Thomas justly remarks, that we have no authority, in 

 our operations, for placing the mean water at Plymouth and 

 at Axmouth at the same level, since our leveling did not ex- 

 tend to Plymouth. Accordingly, we have not drawn any 

 inference from this supposed identity of mean level. The 

 Plymouth observations were referred to, as is stated in the 

 Report, merely in order to ascertain that there were no un- 

 estimated inequalities in the tides on that coast at the time 

 of our observations. But if we have as yet no authority, 

 except that of theory, for assuming the mean waters at Ply- 

 mouth and Axmouth to coincide, Mr. Thomas has no au- 

 thority, either from theory or observation, for assuming the 

 high waters to coincide. And all his other inferences from high 

 waters and from low waters, respecting the level of the sea, 

 are equally gratuitous. 



For the reasons stated here and in the Report, I consider 

 it as certain, that on all open coasts the heights of mean water 

 form a level surface. But I entirely agree with Mr. Thomas, 

 that an extension of this leveling operation to other points of 

 the coast would be desirable. In the conclusion of my Re- 

 port, I have stated several other advantages, besides the con- 

 firmation of the above doctrine, which would be placed with- 

 in our reach by such an extended system of levels. It must, 

 however, be ob vie jus to every one, that such an operation, con- 

 ducted with that care and exactness which alone could render 

 it valuable, would be business of very serious trouble and ex- 

 pense. To have afforded one good example of such an opera- 



