ON EITEES AND ESTUARIES. 557 



from a sandy bottom on to a clean or grassy rocky bottom, it gradually 

 loses its cbarge silting up the bottom. 



The direction in which the sand is moved by the water is sensibly In 

 the direction in which the water which holds the charge is moving. But, 

 as was first pointed out by Dr. James Thomson as affording an explana- 

 tion of the generally observed fact that the beds of rivers are scoured 

 on their convex sides and silted on their concave, the layers of water 

 adjacent to the bed do not always move in the general direction of the 

 stream. There are often steady cross currents at the bottom, as in the 

 case mentioned, though such cross currents do not exist except under 

 circumstances which may be readily distinguished. The most important 

 of these is that pointed out by Dr. Thomson — curvature in the general 

 direction of the stream, in which case the centrifugal force of the more 

 rapidly moving water above overbalances that of the water retarded 

 by the bottom, and forces the latter back towards the centre of the curve. 



This action is universal, where even the lateral boundaries are such as 

 to require the water to move in curved streams ; the drift at the bottom 

 does not follow the general direction of the stream, but sets towards the 

 centre of the curve. 



The result of the foregoing consideration is to lead to the conclusion 

 that the regime of each part of the bed as to maintenance in steady con- 

 dition, lowering or raising it any time, depends solely on the character of 

 the motion of the water, which if straight and uniform, neither acquiring 

 nor losing velocity, causes a uniform drift in the direction of the stream, 

 which maintains the condition steady. If losing velocity the motion causes 

 a depositing drift and raises the bed ; if gaining velocity it causes a scour- 

 ing drift and lowers the bed ; while if curved, the direction of the drift is 

 diverted towards the centre of the curvo with its attendant effect to 

 lower the convex side and raise the concave side of the bed. This con- 

 clusion seems to be of the utmost impoi'tance in dealing with this subject. 

 For if it is correct, not only can the character of the action going on at 

 the bed be inferred from the observed motion of the water, and vice versa, 

 but since, according to this conclusion, the character of the action is inde- 

 pendent of the magnitude or velocity of the stream, the results will be 

 the same on a small scale as on a large one, jjrovided only that the 

 character of the motion of the water is the same at all points. In this 

 latter respect this conclusion affords an explanation of a fact that 

 cannot fail to have struck everyone v.^ho has observed the sand-beds of 

 the streams running over sands which have been left by the tide, viz., 

 what an almost exact resemblance they bear to each other, whether 

 having the size of a moderate river or of the smallest rivulet. 



On the large scale of actual estuaries we can only test the conclusion 

 by actual observation, but on a small scale we can experimentalise in 

 whatever condition of motion we want to test, and readily observe the 

 effects produced ; a possibility of which great use has been made iu this 

 investigation, and which will be again referred to. 



As applied to a non-tidal river, in which the direction of the motion 

 is always the same, the foregoing conclusion would lead us to expect that 

 the regime would be steady except at the bends, the sources, and the 

 mouth, which is exactly what is observed, so that the conclusion so far 

 agrees with experience. The most striking feature about rivers is the 

 way they wriggle about in the alluvial valleys ; a phenomenon pointed 

 to by Lyell as one of those causes still in progress which had produced 



