REPORT ON HYDRAULICS. PART II. 



485 



removed, the number of lakes reduced, and the channel deepened 

 and improved in various parts, notwithstanding which it is gene- 

 rally believed that the navigation would only be fit for steam- 

 boats. . 



The difficulty experienced in preserving the channels of rivers 

 free from the changes which take place in times of flood by the 

 depositions of gravel and other obstacles, induced Col. Burgoyne 

 to make the following statement in his Report : " It is a very 

 usual opinion among engineers, that side artificial canals are 

 eventually more judicious than the attempt to dredge channels 

 in the beds of the streams themselves; and that for the purposes 

 of navigation, rivers are only useful to supply canals with water. 



" It may therefore be important, in estimating the propriety of 

 excavating the required depth of this river according to the plan 

 now proposed, to take into consideration the nature of the shoals. 

 If they have been created by deposits collected by the action of 

 the current, it may be inferred that the same process will con- 

 tinually tend to the same results, and that an effort to preserve 

 the channel wovild require to be constant and laborious ; but if 

 the obstructions have been artificially made, or consist of a 

 natural and solid substratum, it may be reasonable to presume 

 that the openings once made will be permanent, or at all events 

 require but little attention to maintain. Appearances would 

 seem to indicate that the shoals in the Shannon come almost 

 all under the two latter descriptions." 



The Report of the Parliamentary Committee on this sub- 

 ject, in 1834, states, that "great detriment has arisen to the 

 navigation from the land-floods, so prevalent upon the river, 

 and over which there is no machinery for exercising any 

 control." 



The main question appears to be, whether the free and natural 

 flow of the floods is to be arrested by locks, dams, and other 

 works ? 



Although the principles which have guided the operations of 

 our engineers have been various and contradictory, in general 

 the practice has been to confine the freshes by artificial works, 

 as in the Clyde, Witham, and other rivers, and to preserve the 

 receptacles for tidal waters to their full extent. A contrary 

 proceeding has tended to ruin many of our rivers and estuaries, 

 whereby the drainage and navigation have been greatly im- 

 peded, and the destruction of several of our harbours, such as 

 the Dee and Rye, occasioned. 



The effects of embankments in Plymouth and Portsmouth 

 harbours, and particularly in the estuary of the Mersey, (one 

 third of the ancient capacity of which has been filled up by en- 



