THE PIROROCO. 
131 
1849.] 
tact 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 snfiicient 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 repre- 
sents 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 propa- 
gated 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 pro- 
duce a bore, otherwise it would occur much more fre- 
quently 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 
K 2 
