MEASUREMENT OF SEDIMENTARY DISCHARGE OF RIVERS. 219 
carried down annually by a stream and divide this into the cubical con- 
tents of the basin excavated by its action, we should have as a quotient 
the-number of years the operation took to accomplish. Of course many 
disturbing elements enter into the calculation to complicate it and render 
it more or less uncertain; and these must not be lost sight of. It may 
be that the average rainfall of past ages was very different from the present, 
or that climate had a different effect in modifying atmospheric action. 
Still, as a step towards removing our difficulty in the way of cyclic com- 
putation, to estimate the sedimentary contents of rivers is a problem of 
considerable importance. 
The elements required for this calculation are, the quantity of water 
passing a certain spot in a given time, and the quantity of sediment in a 
given bulk of such water. 
The mode I adopted to ascertain the first was as follows. Having 
chosen a reach of the river as straight and as free from obstructions as 
possible, I erected a post, painted white, and divided into feet and decimals 
of a foot, zero corresponding with summer leyel. By this was regis- 
tered from time to time the height of the river. Along the bank a space 
of one hundred feet was measured off and three cross sections made, one 
at each end and the other in the middle of the measured space. From 
these it was easy to construct a mean section. When it was desired to 
note the volume of discharge, the speed of the surface was ascertained 
by throwing in small pieces of wood and marking the time that elapsed 
while they passed the measured hundred feet. Then by the use of certain 
tables, to be found in Neville’s work on hydraulics, it was possible to 
arrive at the mean quantity of water that passed this spot per minute. 
It is necessary previously to construct a table from the data furnished 
by the average cross section, which gives the wetted surface of the river 
bed for each decimal mark on the gauge. 
So much for the volume of water. The amount of sedimentary deposit 
held in suspension was determined by taking at intervals of the flood 
measured bottles of water, then allowing the mud to subside, which it 
sometimes required two or three days to do completely, decanting off the 
clear liquid, carefully drying the residuum on weighed filter paper, and 
afterwards weighing the whole in a balance which indicates correctly to 
the 50th of a grain. The solid matter contained in a given quantity of 
water was thus determined, and the remaining calculation was easy. 
The object of registering the height of the stream on the post or 
gauge is to save the necessity of repeated observations on its velocity. 
The speed varies, of course, with the height, but is constant at any par- 
ticular height. I found that by ruling a sheet of paper with lines at 
right angles and at equal intervals, thus covering it with a number of 
small squares, the divisions in one direction expressing equal heights on 
the gauge, and those in the other the speed of the stream measured in 
seconds and feet, and marking on these lines a number of observations, 
the curve of a rectangular hyperbola was traced, which enabled me 
to construct a table giving the mean volume of water proximately at any 
time when an observation had been missed, when, as during the dark- 
ness of the night, it would have been impossible to make it. 
