L. MECHANICS AND ENGINEERING. 469 



front, foot of Palmer Street. The dimensions of the dock are 

 462 by 111 feet. It has required 4200 piles, and the basin is, 

 of course, made water-tight by sheet piling. The pumping 

 capacity, to be supplied with four centrifugal pumps, pos- 

 sesses an aggregate water-lifting power of 120,000 gallons 

 a minute. The dock will accommodate ships of the largest 

 size, being 100 feet longer than the ships of the American 

 line. 



THE EFFECTS OF LEVEES. 



According: to the Board of Commissioners on the reclama- 

 tion of the alluvial basin of the Mississippi River, the idea is 

 utterly without any good foundation, either in theory or ex- 

 perience, that the deposit made on the river banks must set- 

 tle on the bottom, and that thus the effect of embanking a 

 river is to confine its sedimentary matter to the channel, 

 ultimately raising the bed, and with it the high-water mark. 

 This idea is usually defended by appealing to the example of 

 the Po, in Italy, which river is asserted to have raised its bed 

 several feet. In point of fact, such is not the case. It has 

 been shown in the most conclusive manner, by the great Ital- 

 ian hydraulic engineer, Lombardini, that there is no ground 

 whatever for believing that levees ever produced the slight- 

 est elevation of the bed of the Po. The same truth is dem- 

 onstrated by eighty years of observations upon the River 

 Rhone, and by careful measurements upon the jMississippi. 

 Another theory, diametrically opposed to this, is equally er- 

 roneous, at least as far as regards the Mississippi. It is assert- 

 ed in the most confident manner that that river is flowing in 

 a bed composed of its own deposit, with dimensions regulated 

 in accordance with its own needs, and hence, with the in- 

 creased velocity resulting from the confinement of its volume 

 between levees, will rapidly excavate this bed to a correspond- 

 ing greater depth, thus avoiding any permanent increase in 

 its hiofh-water mark. After reviewincj various considerations, 

 the committee state that we are to conclude that there is no 

 mysterious agency which may be accepted as exerting a con- 

 trolling influence upon the levee system ; and by considering 

 what experience has taught respecting the levees of the Mis- 

 sissippi during the past century and a half, it seems certain 

 that the alluvial regions of that river can only be reclaimed 



