24 Hydraulics of the Mississippi River. 
made at frequent intervals, in a line at right angles to the stream. 
The place of each sounding was fixed by observation with two 
theodolites, from the extremities of a base-line of from four hun- 
dred to one thousand feet in length, measured upon the bank. — 
Two independent sections were sounded, two hundred feet apart; 
and soundings repeated on the same lines, at different intervals 
of time, showed that the bed underwent no sensible changes. 
Lines of level were also run up at the banks to points above the 
highest floods. From these data, with the daily gauge reading, 
showing the stand of the river, the cross-section was known for 
every da 
V cece both at the surface and beneath it, were ascertained 
by means of floats. In a river of such depth and power as the — 
ississippi, No kind of current-meter, or other contrivance in- 
volving mechanism, is available for sub-surface observations on 
velocity. The submerged floats were connected with surface — 
oats very much smaller, by means of cords. The sastane floats — 
dapried small flags which were observed in their transit across 
two lines at right angles to the river, two hundred feet apart, by 
means of theodolites at the extremities of a base of the same | 
length on the shore. The point in which each float crossed each — 
‘section line was thus fi xed. As many observations as possible 
division was taken as — nting the mean velocity of that : 
division. For the shore divisions, ce the floats were not 
distributed through them, a slight correction was sometimes a 
These mean velocities were then multiplied by the areas — 
of their respective divisions of the cross-section; and the sum — 
of the products was divided by the area of the entire cross-See 
tion, ra Ce aie velocity of the river. 
by being multipled into the ratio between the Eibe 
on | pi, and ne mean velocity in the entire vertical plane 
lel to the current. This se had not been discovered while 
ahh baalies anieatcnie te were 
r the Sagi ee of the 2 lawot of velocities below the sur 
. eh of obs seek Mma ton 
