556 KEPOET — 1887. 



simple action of deposit or removal, as they are always attended with the 

 same evidence of the drifting of the surface, and are clearly the result of 

 a diiFerence in the quantities of sand deposited or removed by the drift. 



Movement of ivater. — The manner in which a current of water acts on 

 the granular material forming the bed of the current has been the subject 

 of an investigation by various experimenters. It has been found that the 

 primary action is not so much to drag the grains along the bottom, but 

 to pick them up, hold them in a kind of eddying suspension, at a greater 

 or less height above the bed, for a certain distance and then drop them, 

 so that when the water is drifting the sand there is a layer of water 

 adjacent to the bottom of a greater or less thickness charged to a greater 

 or less extent with sand. The faster the current and the finer the sand 

 the greater will be the thickness of the charged layer, as well as the 

 denser is the charge in the layer. 



A certain definite velocity, according to the size and weigbt of the 

 grains, is required before the water will raise the grains from the bottom, 

 and for all velocities above the minimum necessary to raise the sand the 

 suspended charge increases with the velocity, and the rate of drift or 

 the quantity of sand which passes a particular section increases much 

 faster than the velocity. Attempts have been made with greater or less 

 success to determine exact laws connecting the minimum velocities at 

 which the sand begins to drift with the weight of the grains and other 

 circumstances ; also to determine the exact law of the rate of increase of 

 the drift with the velocity. 



For my present purpose, however, it is not necessary to enter upon 

 such considerations. 



From the facts already mentioned, it will appear that the effect of a 

 uniform current of water over a uniform bed of sand will not be to raise 

 or lower the bed ; for, as the charge of sand in the water remains uniform, 

 it must drop as many particles as it raises everywhere on the bed. This 

 is the action of the water in causing a uniform drift. 



It is also evident that, if the charge in the water as it comes to any 

 particular place is less than the full charge due to its velocity, it will 

 pick up from that place more sand than it drops, and so increase its 

 charge at the expense of the bed, which will there be scoured or lowered. 

 And conversely, if the water as it arrives at any place is overcharged, 

 it will relieve itself by depositing more than it picks up, and so raise or 

 silt up the bed. 



As regards the circumstances which can cause the water to be charged 

 to a greater or less extent than that which it would just maintain with 

 such velocity as it has, the most important are — 



(1.) An increasing or diminishing velocity. When the water is 

 moving in a stream from a point where the velocity is less to one where 

 it is greater, the velocity of the actual water as it moves along is increas- 

 ing, as will also be its normal charge of sand ; hence it must be continnally 

 picking up more than it deposits. And conversely, when moving from a 

 point of greater velocity to one of less, its normal charge will be continu- 

 ally diminishing through deposits on the bed. 



(2.) Another circumstance whicb affects the charge of sand witb 

 which the water may arrive at a particular point is a variation in the 

 character of the bed. If, for instance, water flows from a rocky bed on 

 to sand, it may arrive on the sand without charge, and immediately 

 charges itself at the expense of the bed. Or again, where water flows 



