found ly the French in Upper Egypt, 375 



for founding this depth than the refill t of fuch calculations. 

 Thefe are, the numerous volcanic ides fcattered in all lati- 

 tudes throughout the bofom of the moft ex ten five feas. Thefe 

 iflands atteft, that the bafis on which the fubterranean fires 

 have elevated them cannot be at the depth of even a thoufand 

 toifes ; and if we add to thefe volcanic ifles all the natural 

 ifles which are at a great diftance from continents, what will 

 become of the depth of four leagues, afligned to the fea from 

 thefe calculations ? 



Another fact, which oppofes this hypothefis, refults from 

 currents. If the flux and reflux depended on the depth of 

 the fea, it ouo;ht to experience the fame librarian throughout 

 its whole mafs ; which is not the cafe. The currents are not 

 fenfible but at a fmall diltance from the furface of the water. 



This fact, which is well known to navigators, fupplies 

 them with the means of determining whether their veflel be 

 in a current. They hoift out a boat, which proceeds to fome 

 diftance from the veflel, and then let down a weight attached 

 to a rope to the depth of 200 fathoms. This weight being 

 thus at a great depth in calm water, obfervation and experi- 

 ence having fhown that currents are not fenfible beyond the 

 depth of ten fathoms, it produces the effect of an anchor 

 which retains the boat: they then throw into the water a 

 very thin board, that the wind may have no hold of it, and 

 according to the motion of this board, if it has any, they 

 difcover whether there be a current, and determine its direc- 

 tion and velocity. It refults then from thefe fads, that the 

 libration of the fea, occafioned by the moon, which produces 

 the tides, is owing to its extent, and in no manner to its depth. 



What then ought we to conclude from all thefe facts? 

 That in regard to the philol'ophy of the earth we ought to 

 confult them, and direct our observations in particular by the 

 firft of all motives, that of attaining, as far as poflible, to the 

 knowledge of truth. 



I (hall take this opportunity of making a remark connected 

 with fome obfervations contained in my letter of the 10th of 

 November laft. Since the currents of the fea are experienced 

 only at a fmall depth, and as the fea beneath them is calm, 

 the geological fyflems founded on the formation of new con- 

 tinents by currents at the bottom ot the fea have not this 

 fuppofed fact in their favour, fince it is proved that they do 

 not exifl. 



Geneva, 

 May 10, 1S02. 



Aa4 LXTII. Ob- 



