Mathematical and Physical Science. S23 



bours and experience arrived. One method proposed was, that 

 marks should be made along- various parts of the coast, which 

 marks should be referred to the level of the sea ; but here the in- 

 quiry met us in the very outset — what is the proper and precise 

 notion to be attached to the phrase the level of the sea? Was it 

 high water-mark or low water-mark ? Was it at the level of the 

 mean tide, which recent researches seemed to establish ? In hydro- 

 graphical subjects the level of the sea was taken from low water, 

 and this, although in many respects inconvenient, could not yet be 

 dispensed with, for many reasons, one of which he might glance at 

 — that by its adoption, shoals, which were dry at low water, were 

 capable of being represented upon the maps as well as the land. 

 The second method proposed appeared to be one from which the 

 most important and conclusive results were to be expected. It 

 consisted in accurately levelling, by land survey, lines in various 

 directions, and by permanently fixing, in various places, nume- 

 rous marks of similar levels at the time ; by the aid of these 

 marks, at future periods, it could be ascertained whether or not 

 the levels, in particular places, had or had not changed, and thus 

 the question would be settled whether or not the land in parti- 

 cular localities was rising or falling. Still further, by running on 

 those lines, which would have some resemblance to the isother- 

 mal lines of Humboldt, as far as the sea coast, and marking their 

 extremities along the coast, a solution would at length be obtained 

 to that most important practical question, — what is the proper or 

 permanent level of the sea at a given place? Until something 

 like this were accomplished, Mi* Whewell expressed his strong 

 conviction of the hopelessness of expecting anything like accu- 

 racy in many important and even practical cases. As an ex- 

 ample, he supposed the question to be the altitude of Dunbury 

 Hill referred to the level of the sea : if that level of the sea were 

 taken at Bristol, where the tide rises, as before stated, fifty feet, 

 the level of low water would differ from the same level on the 

 sea coast at Devonshire, where the sea rises, say eighteen feet ; 

 and supposing, as is most probable, the place of mean tide to be 

 the true permanent level by no less a quantity than sixteen feet, 

 which would therefore make that hill to appear sixteen feet higher, 

 upon a hydro graphical map constructed by a person taking his level 

 from the coast of Devonshire, than it would appear upon the map 

 of an engineer taking his level at Bristol. In the method pro- 



