90 TRAVELS ON THE AMAZON, [July, 



water is put in motion : the depth of the moving water is less, 

 and does not come in contact with the bottom in passing over 

 the shoal, and so no wave is formed. It is only when the 

 body of water in motion, as the tide first flows in, is of 

 sufficient depth, that 4 it comes in contact with the shoal, and 

 is, as it were, lifted up by it, forming a great rolling wave. 



The above diagram will show more clearly the manner in 

 which I suppose the wave to the formed. A A represents the 

 level of the water when the tide is out ; D D the bottom of 

 the river ; B B the depth to which the water is put in motion 

 at low tides, not reaching so deep as the bottom of the river 

 at the shoal C, at which time no wave, but a swift current 

 only, is formed ; C' C the depth to which the water is set in 

 motion at spring-tides, when the mass, coming in contact with 

 the bottom at C, is lifted up, and forms a wave at E, which is 

 propagated up the river. It appears, therefore, that there must 

 exist some peculiar formation of the bottom, and not merely 

 a narrowing and widening in a tidal river to produce a bore, 

 otherwise it would occur much more frequently than it does. 

 In the Moju and Acarra the same phenomenon is said to take 

 place ; and, as these rivers all run parallel to each other, it is 

 probable that the same bed of rock running across produces 

 a somewhat similar shoal in all of them. It may also easily 

 be seen why there is only one wave, not a succession of them ; 

 for, when the first wave has passed, the water has risen so 

 much that the stream now flows clear over the shoal, and is 

 therefore not affected by it. 



On arriving at Para I again took up my abode at Nazare. 

 I had found in this voyage that my canoe was far too unsteady 

 and confined to think of going up the Amazon in it, so I 



