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Miscellaneous Intelligence. 155 
of the Mississippi River. The general results obtained by the two 
methods of observation are also compared, and the points of disa- 
greement considered. An Appendix contains a reply to the criti- 
cisms, published in the annual report of the Chief Engineer, U.S. A., 
to the Secretary of War for 1870, on the author’s methods of current 
easurement, as set forth in his “ Progress Reports, submitted to 
the Superintendent of the Lake Survey ;” and a Mathematical 
Investigation on the use of floats in Guaging Rivers and Streams, 
by the distinguished mathematician, Prof. 8. W. Robinson, of the 
Illinois Industrial Institute. 
The author maintains that the nt of sub-surface veloci- 
ties by the method of double floats is liable to the following 
“principal causes of error,” which do not affect measurements 
made with the telegraphic meter. 
(1.) The error of cross section ; which arises from the fact that 
“in order to ascertain the true mean area, it would be necessary 
o know the exact depth of the river past the whole base line; 
but generally it is considered sufficient to sound out two or three 
nes across the river, and take their mean depth, which, when the 
bottom is not perfectly regular, may differ considerably from the 
true mean depth.” 
_(2.) The pulsation of the current ; or, the existence, under all 
circumstances, of “an intermittent velocity, increasing and dimin- 
ishing in accordance with some yet undiscovered law ;” which 
may materially alter the result obtained by the double float, while 
all its irregularities will be accurately recorded by the telegraphic 
meter. 
(3.) The uncertainty of location of the points at which the 
float crosses the upper and lower transverse section lines of the 
stream 
(4.) Floating bodies move faster than the water in which they 
pote immersed. “This error is very small compared with the 
others.” é 
(5.) The upper float drags the lower. “This error is also small 
and depends upon the relative size of the floats, and the velocity 
of the current.” 
