and Orography of the Earth's Surface. 309 



When the geographical value of the extent of land and 

 water is known, it is possible then to determine by batho- 

 graphy and geodesy the extent of the waters of all seas, as 

 well as that of the protuberances of our earth spheroid. 



These numbers got, we could establish with them a normal 

 medium for the thickness of the last covering of the rigid 

 part of the globe, which forms especially now the continents 

 and heights ; in the meantime, one would deduce from the 

 extent of the fluid, the medial height with which this water 

 once surrounded the rigid part. On this base all the changes 

 known would have followed, and we could estimate all the 

 values of subsidences and elevations. 



Afterwards one would determine exactly the geographical 

 surface and space of the continents in each great geological 

 period, to get the value of the place and space occupied during 

 the same times by the water. To replace the surface of the 

 land which probably was lost by subsidence in some geolo- 

 gical periods, one should employ the probability of calcula- 

 tions which may be based on what remained from each period, 

 on the mode of distribution of continents from the beginning 

 till now. But an absolute necessity would always remain, 

 viz., the knowledge of the greatness of each series of eleva- 

 tions in each period. To get this, it is only necessary to 

 make the following reasoning. As we know now the mutual 

 relation of the surface of the actual seas to that of land, as 

 well as to what these were in the alluvial period, we can 

 then conclude what surface the sea covered in the tertiary 

 time ; we must subtract from the value of the surface of land 

 in the alluvial period, that which it had in the tertiary, and 

 add this difference to the sum of the surface value of the sea 

 in the old alluvial period. 



But when two seas of the kind have not the same surface 

 value, the smaller must replace the want of space by the 

 greater depth. This necessity is the best proof that the seas 

 have gained in depth from the oldest time till now, and that 

 in exact proportion as the land became always greater and 

 greater in extent. First there existed only islands, and for 

 that reason a shallow sea ; the more this extended, the deeper 

 the sea became. 



