330 THE POPULAR SCIENCE MONTHLY. 



nation either by the curvature of the banks, or by some physical irreg- 

 ularity of the channel." It was determined by actual experiment 

 that the greatest velocity of current is at the surface and the least at 

 the bottom, and that tlie increase of velocity " is in the simple ratio 

 of the distance from the bottom." This decides that the mean ve- 

 locity of a stream is to be found at half its depth. A result perfectly 

 consistent with the previously expressed law that surface velocity is 

 proportionate to depth, it is in fact a corollary, and one that was veri- 

 fied by experiment. 



For determining the velocity of a stream, M. Revy employed an 

 instrument called the current-metre. This consisted of a propeller- 

 like wheel attached to a long axle and made to turn witli rapidity 

 when immersed in a running stream. The wheel is fastened to a kind 

 of rudder which keeps it always against the stream. The upper end 

 of the axle moves a set of cogs which turn a couple of indices upon a 

 dial-plate. M. Revy is not the inventor of the instrument, but he im- 

 proved it so as to greatly increase its utility. He first ascertained the 

 correct reading of the metre by moving it through still water, and 

 next made an extensible addition to the axle, so that the instrument 

 could be adjusted to any depth. 



An interesting verification of M. Revy's discoveries with regard to 

 the velocity of currents was unexpectedly made. " In two successive 

 measurements of the current of the La Plata, he found that there was 

 a decrease of surface velocity and an accompanying increase of lower 

 velocity. This at first seemed to contradict his previously-obtained 

 results, but on further examination it was discovered that the depth 

 at those points was sixteen inches greater. The water was banked up 

 by its own mass so that the surface fall was less than at the point of 

 previous measurements. "In obedience to known hydraulic law, this , 

 decreased surface-inclination was indicated by decreased velocity of 

 surface-current. But the power of the whole moving mass was greater 

 in proportion to its depth. And thus, while to the superficial gaze the 

 velocity was less, the mean velocity was greater; and the river swept 

 with more resistless energy over its bed. This luminous observation 

 is the opening of an entirely new chapter in hydraulic science. It 

 is, in fact, a case of the law of relation of speed to depth ; but it is 

 one that could scarcely have been arrived at by theory; although, 

 now that it is experimentally ascertained, its theoretic reason is ascer- 

 tainable." 



These results, of course, require further verification elsewhere, but 

 they cannot fail to be of the highest value both to experimental and 

 practical science, for they will at any rate furnish method, and give 

 new impetus to hydraulic investigation. Should they be verified, and 

 they probably will be, it will follow that wherever water-power is de- 

 sired, as in mill-races, or abundance of water-supply, as in aqueducts, 

 depth of way should be secured at the expense of breadth. 



