130 Up the Congo to Banza Nokki. 
three to five knots in rare places ; the surface loses 
the glassiness of the lower section, and at once 
shows the boiling and swirling which will be 
noticed near the cataracts. The shores are often 
foul, but the midway is mostly clear, and, where 
sunken rocks, are, they are shown by whirlpools. 
The flow of the tide, or rather the damming up of 
the lower waters between Porto da Lenha and the 
mouth, causes a daily rise, which we found to 
measure about a foot; thus it assists in forming a 
treble current, the rapid down-flow in the Thalweg 
being subtended by a strong backwater on either 
side carrying a considerable portion in a retrograde 
direction, and showing a sensible reflux; this will 
continue as far as the rapids. In the Amazonas 
the tides are felt a hundred leagues from the 
mouth ; and, whilst the stream moves seawards, 
the level of the water rises, proving an evident 
under-current. Mr. Bates has detected the in¬ 
fluence of oceanic tides at a point on the Tapajos, 
530 miles distant from its mouth, such is the 
amazing flatness of the country’s profile: here we 
find the reverse. 
The riverine trough acts as wind-conductor to a 
strong and even violent sea-breeze ; on the lower 
section it begins as a ground-current—if the “ bull ” 
be allowed—a thin horizontal stratum near the 
water, it gradually curves and slides upwards as it 
meets the mountain flanks, forming an inverted 
