304 M. Ami Boue on the Palceohydrography 



i 

 in the North American point, we can estimate the value of 



the low shore, when we know the elevation of the high and 

 steep chain, but the sea should have on both sides the same 

 depth, which is frequently not the case. The sea can be deep 

 on one side, and shallow on the other, or deep or shallow on 

 both. For this reason, the normal depth of the sea will al- 

 ways better suit for the calculations. 



When a rigid part of the earth was elevated, vaults were 

 produced, or in other words, elevations and subsidences, ac- 

 cording to the principle of the see-saw motion. If the value 

 of such an elevation above the level of the sea is found, it is 

 easy to obtain that of the subsidences under water, because 

 both values are determined by an equal angle around a fixed 

 point. A country might have been subjected to a simple 

 see-saw like motion, as England for instance, where one 

 shore is high and hilly, and the other flat, with subsidences 

 in the Northern sea. 



The middle of an island can have been vaulted with a kind 

 of double see-saw motion, of which the two elevated extre- 

 mities represent the middle of the vault. The subsidences 

 of both sides under the sea-level would equal the height of 

 the vault above the sea-level. 



The variation in the position of the highest part of the 

 elevation changes nothing in the results, the triangles which 

 are to be constructed on both sides, above and below the sea- 

 level, will only become more and more unequal the more the 

 greatest elevation is placed further from the middle part of 

 the observed land on one or other side. 



If two tijjangles represent the vault above the sea-level, and 

 their base be that level, and if we lengthen these lines on 

 both sides of their relative value in the triangles, and if we 

 do the same with the two lines which descend from the 

 middle of the vault, till the sea-shore on both sides become, 

 through this construction on each side under the sea, similar 

 triangles with angles of equal value as the 



vault above the sea. This result remains 



the same whatever irregularity the vault V/ 



may have ; but in the last case the values of the triangles 



and a gics on both sides are unequal. In this way we arrive 



