236 



THE POPULAR SCIENCE MONTHLY 



CORRESPONDENCE. 



RIVER HYDRAULICS. 



To the Editor of The Popular Science Montldy : 



IN the July number of The Popular Sci- 

 ence Monthly, there is an article en- 

 titled " The Hydraulics of Great Rivers," 

 said to have been mainly derived from an 

 account, in the April number of the Edin- 

 burgh Review, of a book called " The Pa- 

 rana, the Uruguay, and the La Plata Estua- 

 ries," by M. Revy, a member of the Insti- 

 tute of Civil Engineers of Vienna. 



In that article there were so many state- 

 ments calculated to give an erroneous im- 

 pression to the reader, that it seemed to me 

 some comment was called for. These state- 

 ments I will examine in turn : 



" At the point near Rosario, where the 

 river is 4,787 feet wide, a series of meas- 

 urements has been made by M. Revy, which 

 constitutes the largest measurement of a 

 river section yet effected. . . . The average 

 depth was 47-J- feet, and the greatest 72 feet, 

 while the sectional area measured 184,858 

 feet." 



In the " Report upon the Physics and 

 Hydraulics of the Mississippi River," by 

 Captain Humphreys and Lieutenant Abbot, 

 of the Corps of Topographical Engineers, 

 United States Army, published by Lippin- 

 cott & Co., in 1861, by authority of the 

 War Department, there are given the di- 

 mensions of 93 cross-sections of the Mis- 

 sissippi River, commencing at Columbus, 

 Ky., and extending to Fort St. Philip, some 

 75 miles below the city of New Orleans. 

 Two sections, one at Osceola, Ark., the 

 other at Randolph, Tenn., measured by 

 Lieutenant Abbot in 1858, are, respective- 

 ly, 6,880 and 6,080 feet in width, 195,844 

 and 184,717 square feet in area, the maxi- 

 mum depths being 87 and 117 feet; while 

 four of the other sections of the Mississippi 

 exceed 243,000 square feet in area, which 

 M. Revy gives as the measurement of the 

 same Rosario section during the ordinary 

 flood. The claim, therefore, of " the largest 

 measurement of a river-section yet effect- 

 ed," is hardly a valid one. The language, 



" largest measurement yet effected," rather 

 gives one the idea that it is a pretty diffi- 

 cult matter to measure the cross-section 

 of a large river, whereas it is a compara- 

 tively simple operation. A sounding-party 

 in a boat, and two observers at the extrem- 

 ities of a carefully-measured base-line on 

 shore with theodolites taking simultaneous 

 observations on the sounding-boat, are all 

 that is necessary to determine the section 

 with great accuracy. 



To quote again : " While, therefore, it 

 is easy to measure the velocity of the sur- 

 face-current, it is difficult, because of this 

 retardation beneath, to determine the mean 

 velocity or actual flow of the river. This 

 has never been satisfactorily done before. 

 Many experiments, with a view to the ac- 

 complishment of this end, have indeed been 

 made by eminent men, but they have failed 

 to establish the relationship between the 

 depth of the stream and the velocity of the 

 flow. M. Revy has established that the 

 velocity of a river is directly proportion- 

 ate to its depth, diminishing or increasing 

 therewith." 



It is indeed true that many eminent 

 men have occupied themselves with this 

 problem ; but whether they have failed to 

 establish the relation between the velocity 

 and depth is another question, though they 

 certainly failed to find the relation so ex- 

 tremely simple as that determined by M. 

 Revy. In the Government publication re- 

 ferred to, there is an outline of the history 

 of hydraulics applied to rivers, beginning 

 with the contributions to the subject by 

 Castelli, a pupil of Galileo, in 1628, and 

 extending up to the date of the Report. 

 And, by-the-way, Dr. Thomas Young, a por- 

 trait and biographical sketch of whom are 

 in the same July number of The Popular 

 Science Monthly, was a contributor to this 

 subject. The numerous formula? of different 

 investigators are given in the Report, all re- 

 duced to a uniform system of notation, but, 

 are rather too complicated to be merely 

 copied in a letter, without the accompany- 



