506 Mr. L. F. Veriion-Harconrt. 



involved, which differ more or less in each case, and thus render it 

 difficult to lay down general rules for guidance from arguments based 

 on analogy. One of the most important considerations is the form of 

 the estuary ; and in this respect no two estuaries are alike, as their 

 form is the result of complex geological and hydrological conditions ; 

 and it suffices to contrast the Mersey and the Rihble, the Dee and the 

 Tay, the Clyde and the Tees, the Seine and the Loire, to indicate the 

 varieties of forms which may have to be dealt with. Other circum- 

 stances affecting the problem are the rise of tide, the tidal capacity 

 and general depth, the fresh-water discharge, the silt introduced by 

 the flood tide or brought down by the river, the condition of the sea 

 bottom in front of the mouth, and the direction in which the tidal 

 current enters the estuary. The positions also of ports established at 

 the sides of estuaries require special consideration in determining 

 the proper line for a trained channel. These numerous and variable 

 conditions have often led engineers to enunciate the opinion that each 

 river must be considered independently by itself. This view, how- 

 ever, if strictly adhered to, by excluding the experience derived from 

 previous works, would prevent any progress in the determination of 

 general principles for the improvement of navigation channels through 

 estuaries ; each training work would form an independent scheme, 

 based upon no previous experience, and might or might not produce 

 the results anticipated by its designer. Unfortunately also it is im- 

 possible to proceed with training works by the method of trial and 

 error ; for besides the cost of modifying the lines of training walls, 

 if the desired results are not produced, these works generally effect 

 such extensive changes in an estuary, that it would be impracticable 

 to restore the original conditions, or to modify materially the altered 

 position. 



It might be possible to deduce general rules for training works 

 from a careful consideration of a variety of types of estuaries, espe- 

 cially those in which training works have been carried out ; and I 

 have commenced an investigation of this kind. This method of in- 

 quiry, however, requires a variety of data which it is difficult to obtain 

 for most estuaries, and must depend upon a careful estimate of the 

 relative influence of each of the variable conditions, and a train of 

 reasoning from analogy which might not be accepted by engineers as 

 conclusive. Accordingly, it would be of the very highest value to 

 river engineers, and of considerable interest from a scientific point of 

 view, if a method of investigation could be devised, which might be 

 applied to the special conditions of any estuary, and the results of any 

 scheme of training works determined approximately beforehand, in 

 a manner which could be relied upon from the fact of their depending 

 on an assimilation to the actual conditions of the case investigated, 

 and not on arguments based upon the effects of similar works under 



