VOLUME OF RIVERS. 57 
average velocity of the surface of the current, which is greater than that 
of its entire body, owing to frictional retardation at the bottom. 
To make the necessary measurements, choose a place where the river 
runs steadily in a straight and deep channel, and where a boat can be 
had. Prepare a few floats of dry bushes with paper flags, and be assured 
they will act. Post an assistant on the river-bank, at a measured dis¬ 
tance, of about half the estimated width of the river, down stream, in face 
of a well-marked object. Row across stream in a straight line, keeping- 
two objects on a line in order to maintain your course. Sound at intervals 
from shore to shore, fixing your position on each occasion, by a sextant - 
angle between your starting-place and your assistant’s station, and throw 
the floats overboard, signalling to your assistant when you do so, that he 
may note the interval that elapses before they severally arrive opposite to 
him. Take an angle from the opposite shore, to give the breadth of the 
river. 
To make the calculation approximately, protract the section of the river 
on a paper ruled to scale in square feet, and count the number of squares 
in the area of the section. Multiply this by the number of feet between 
you and the assistant, and divide by the number of seconds that the floats 
occupied, on an average, in reaching him. 
Important rivers should always be measured above and below their 
confluence; for it settles the question of their relative sizes, and throws 
great light on the rainfall over their respective basins. The sectional 
area at the time of highest water, as shown by marks on the banks, and 
the slope of the bed, ought also to be ascertained. 
Example. 
Start¬ 
Oppo¬ 
Distance from Shore 
ing 
place. 
site 
Shore. 
Whence the boat started, mea-t 
160 
600 
780 
sured in feet.. .. .. .. j 
0 
90 
240 
330 
420 
500 
'JOO 
Depth at those distances jnea 1 
sured in feet.j 
0 
2 
J* 
4 
4 
54 
7 
6 4 
34 
0 
Ave¬ 
Time required for float to drift] 
rage. 
opposite to assistant, mea-i 
sured in seconds.j 
0 
48 
5 ® 
40 
33 
29 
^7 
30 
50 
• 0 
38*4 
Distance of as^istanf, ip feef, 150. 
