MESSRS. EDITORS—Under the heading of " South American Rivers," in a late number of the Scientific American, I have observed the following statement : " three inhes per mile in a smooth straight channel, gives a velocity ot about three miles per hour." The same statement I observed many years ago in the " Pottsville Gazette," and as the statemsnt cannot be true, only upon certain conditions, and no conditions being mentioned, exceptions may be taken to the statement. Th velocity of rivers depends on several circumstances —the fell or rate of descent, the quantity oi water, and the form ot the channel, as well as the OTndifciojMjiimioothriess and straightness. Ths' GangeHBHB said to have a velocity of about tIiBjrier hour wittl a fal1 of only '''yHB"1'1"' but with a mean hydraulic defHBfiy feet. The Ohio river, from BealSjJHpriieelirjg, has a descent ol 9 inches per BHS and yet the velocity of the stream at the Wheeling " bar," where the velocity must be greater than the uniform flow, is less than 2J miles per hour in low water, but its velocity has increased to nearly 17 miles per hour during great floods. The Miami River, in Ohio, which has a fall of a little more than four teetper mile, has a velocity of about 7i miles per hour during ordina ry floods, but in low water I doubt whether the velocity reaches one mile per hour. This is written only because the character of your paper may give more credit te the statement above quoted than it deserves, and consequently lead to error. Da'yfcn, Ohio. D. H. MORRISON. [We certainly believe that smoothness straightness, and the fall, as given in our extract, covers all that Mr. M. speaks of, excepting the quantity of water, which certainly should always be taken into consideration.