198 ON THE MISSISSIPPI. 



ing retardations at the sides as well as at the bottoms of rivers and 

 canals. 



The energy of deep rivers which has been insisted upon by 

 Guglielmini is not entitled to much notice : we must however 

 admit that water, like solid gravitating bodies descending along 

 an inclined plane, will acquire velocity until the accelerations 

 and resistances are in equilibrio, but from its extreme mobility 

 is more liable to lose it : a globe of solid matter rolling along a 

 horizontal plane loses its motion instantaneously on its falling to 

 pieces; it is not therefore astonishing that water, divisible into the 

 minutest parts, descending into every cavity and deflected by the 

 smallest obstacles, should be speedily deprived of its velocity. As 

 a small body impinging with great velocity upon a large mass 

 may communicate no sensible velocity to the compound, so in 

 like manner, a descending torrent being received into a more 

 capacious bed is totally disarmed of its fury and moves on with 

 a new velocity proportioned to the new declivity. 



Deep rivers moving with a certain velocity and meeting with 

 obstacles will exert the energy spoken of by Guglielmini, that 

 is, like all other heavy bodies in motion, they will endeavour 

 to persevere in the right line of their last motion, and the wa- 

 ters will accumulate against the object, having a tendency to 

 rise to the height of a reservoir, which would produce the actual 

 velocity of the current: thus if M. Pitot's tube A B, (Fig. 4,) 

 be set with its horizontal orifice B, against the current, the 

 water will ascend to C, a height proportioned to the velo- 

 city of the current at B; that is, the column C D, pressing 

 above an orifice in any reservoir would produce a velocity in 

 the spouting fluid equal to that of the river at B : this instru- 

 ment may be commodiously used for ascertaining the velocity 

 of currents where great accuracy is not required, and in low 

 velocities; the tube might be graduated so as to give the velo- 

 city by inspection: it may also be used to determine the dif- 

 ference of superficial and inferior currents. Were the theory 

 true which we oppose, a remarkable effect would be seen in 

 Pitot's tube; the water ought to rise in the tube to a height 

 above the surface of the river, equal to the depth at which it is 

 plunged below the surface, and it" the tube be rendered station- 



