61 



not depend solely on the greatness of partial 

 falls ; #hat determines the force and impetuo- 

 sity is the nearness of these falls, the steepness 

 of the rocky dikes, the returning sheets of wa- 

 ter*, which strike against and surmount each 

 other, the form of the islands and shoals, the 

 direction of the counter currents, and the con- 

 traction and sinuosity of the channels through 

 which the waters force a passage between two 

 adjacent levels. In two rivers equally large, 

 that of which the falls have least height may 

 sometimes present the greatest dangers, and the 

 most impetuous movements. 



I have given with hesitation my opinion of 

 the perpendicular height of the raudales of the 

 Oroonoko, limiting it to one extreme quantity. 

 I carried the barometer to the little plain, that 

 surrounds the mission of Atures, and to the 

 cataracts, but I could not obtain any constant 

 differences. Every one knows how delicate a 

 business it is, to measure small heights by the 

 barometer. It would have required an instru- 

 ment, in which the point of nought was not 

 determined by a constant flowing. Little irre- 

 gularities of the horary variation (irregularities 

 that bear more on the quantity of the variation, 

 than on the period) render the results uncer- 



* Bremontier, Recherches sur le Mouvement des Ondes, 1809, 

 § 6. 



