234 Professor Forbes on the Leading Phenomena of Glaciers. 



mental retardations be communicated towards its centre, and 

 the general velocity of the stream will be more nearly regu- 

 lated by the limit of the mobility of its parts. 



III. In every case, the greatest variation of velocity of such 

 a stream will take place near the sides and bottom, whilst the 

 higher and the central parts will move most nearly together.* 

 The position of any particle moving with the mean velocity of 

 the entire stream, has not, I believe, been determined ; but 

 Dubuat has practically found this singularly simple result, that 

 the velocity of the top and bottom of a stream being known, 

 the mean velocity of the entire stream is the arithmetical mean 

 of these two velocities. 



IV. The difference of the velocities of a stream at the top 

 and bottom, depends upon the actual velocity of the stream, 

 and increases as that velocity increases. The rate of increase 

 appears to be as the square root of the velocity, and is inde- 

 pendent of the depth, t 



V. The velocity of the water in a stream increases with its 

 declivity. If the bed of a river be highly inclined, the water 

 flows rapidly ; and again, if the embaying of a river by a strait 

 accumulates the water above, there its declivity will be dimi- 

 nished. 



VI. If any circumstance causes the viscosity or consistence 

 of a fluid to vary, all these phenomena will vary proportionally. 

 Thus, warm water is less viscid than cold, and a vessel will be 

 sooner emptied through a narrow aperture the higher the tem- 

 perature of the liquid. % 



Now, in all these respects, we have an exact analogy with 

 the facts of motion of a glacier, as observed on the Mer de 

 Glace. 



Firsts We have seen that the centre of the glacier moves 

 faster than the sides. We have not indeed extended the 



* A slight consideration will shew, that this might naturally be anticipated, 

 yet some eminent writers have supposed the velocity to increase uniformly from 

 the bottom to the surface of a stream. The doctrine of the text is fully confirmed 

 by direct experiments upon the river Rhine, quoted in Mr Rennie's Report on 

 Hydraulics, Part II., British Association Report, 1834, p. 467. 



t Dubuat, Art. 37, 49, 65. % I^id, Art. 3. 



