202 ELEVATION OF THE RAIJDALES. 



ft longitudinal valley between the mountains of Parima and 

 the great mass of the mountains of Brazil. 



I was surprised to find by actual measurement that the 

 rapids of the Orinoco, the roar of which is heard at the 

 distance of more than a league, and which are so eminently 

 picturesque from the varied appearance of the waters, the 

 palm-trees and the rocks, have not probably, on their whole 

 length, a height of more than twenty-eight feet perpen- 

 dicular. In reflecting on this, we find that it is a great 

 deal for rapids, while it would be very little for a single 

 cataract. The Yellalas of the Bio Congo, in the contracted 

 part of the river from Banza Noki as far as Banza Inga, 

 furnish, between the upper and lower levels, a much more 

 considerable difference; but Mr. Barrow observes, that 

 among the great number of these rapids there is one fall, 

 which alone is thirty feet high. On the other hand, the 

 famous pongos of the river Amazon, so dangerous to go up, 

 the falls of Kentema, of Escurrebragas, and of Mayasi, are 

 but a few feet in perpendicular height. Those who are 

 engaged in hydraulic works know the effect that a bar of 

 eighteen or twenty inches' height produces in a great river. 

 The whirling and tumultuous movement of the water does 

 not depend solely on the greatness of partial falls ; what 

 determines the force and impetuosity is the nearness of 

 these falls, the steepness of the rocky ledges, the returning 

 sheets of water which strike against and surmount each 

 other, the form of the islands and shoals, the direction of 

 the counter-currents, and the contraction 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 move- 

 ments. 



It is probable that the river Orinoco loses part of 

 its waters in the cataracts, not only by increased evapo- 

 ration, caused by the dispersion of minute drops in the 

 atmosphere, but still more by filtrations into the subter- 

 raneous cavities. These losses, however, are not very per- 

 ceptible when we compare the mass of waters entering into 

 the raudal with that which issues out near the mouth of the 

 Itio Anaveni. It was by a similar comparison that the 



