738 MK. K. D. OLDHAM ON THE LAW THAT GOVEETSTS 



channel of 150 feet broad, in which the slope of deposit is 14-5 feet 

 per mile : but, where the stream is allowed to spread, the slope 

 immediately falls to 3*04 feet per mile, and increases after 1 furlong 

 to 20-3 feet per mile on the fan. 



From this it wiU be seen that the hypothesis above propounded 

 leads to conclusions which are strongly supported by the surveys and 

 levels of topographers and engineers, whose object was a mere 

 delineation and record of observed facts without any consideration 

 of their possible bearing on an unmooted hj^pothesis ; and this 

 agreement appears to be as strong a confirmation as could be desired. 



These considerations lead us to regard a flowing stream as another 

 instance of the automatic adjustments of jSTature by which cause is 

 proportioned to effect, and the energy exerted to the work to be done. 

 They moreover show that the ordinary text-book statement, that the 

 size of the debris transported depends on the velocity of the current, 

 is but a partial account of the case ; that though the velocity of the 

 current is directly due to the slope and shape of the channel, these 

 are largely controlled by the nature of the burden cast on to the 

 stream : and that, where equilibrium has been established, where, to 

 borrow a phrase, the stream is in unison with its environment, the 

 velocity of the current may be said to depend on the size of the 

 debris it carries. 



It will be no answer to this to point out that the actual velocity 

 of a stream is often in excess or defect of what the hypothesis 

 demands; for, in such cases, the stream is either eroding or depositing, 

 and the mere fact of erosion or deposition of itself proves that equi- 

 librium has not been, but is being, attained. "Where the velocity is 

 too great, erosion tends to diminish the average gradient, and hence 

 the velocity ; and if, owing to a defect of velocity, deposition takes 

 place, the stream deposits its burden in such a manner as to increase 

 the general gradient to the limit at which the velocity will be 

 suficient for the transport of its burden. 



These local exceptions cannot be held of importance in the face of the 

 broad fact that on the whole it is in the upper reaches of a river, 

 where the burden cast upon it consists of coarse debris, that high 

 velocity is found, while in the lower reaches, where the burden 

 consists of fine mud, a low velocity prevails. And this fact, even in 

 the absence of the special considerations detailed above, might by 

 itself be fairly held to show that the velocity of the stream is ulti- 

 mately regulated by the work it has to do, and not its work by the 

 velocity it possesses. 



Discussion. 



Mr. Deew hoped that Mr. Oldham would continue his observa- 

 tions. The paper was difficult to discuss without being read, but 

 Mr. Drew did not understand there being greater velocity on a " fan," 

 the origin of which implied loss of velocity. Mr. Oldham used the 

 word " fan " in a rather different sense from that in which it was 

 originally proposed by the speaker. 



