Page:A narrative of travels on the Amazon and Rio Negro.djvu/114

 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 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 Ievel 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 Mojú 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 Nazaré. 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