REPORT ON HYDRAULICS. — PART II. 435 



and form sand-banks, which will go on gradually increasing ; 

 and the river being opposed on one or other side, according to 

 the direction of the current of tide, will turn to the right or left 

 as may be. Proposition 3 : — Not only will the depth of the united 

 river be increased, but the depths of all the other affluents 

 likewise. 



The remaining principles attempted to be established in this 

 chapter are : 



1st, That it is improper to unite rivers which carry gravel 

 with rivers which carry sand ; 



2ndly, That the courses of gravelly rivers should not be short- 

 ened towards their embouchures j 



3rdly, That the corrosions of the borders of united rivers are 

 inevitable ; 



4thly, That it is better to cause a river carrying gravel to de- 

 posit its gravel by lengthening its course than to join it with 

 another river carrying sand : that the consequences of such a 

 junction would be to oblige the greater river to change its direc- 

 tion or to raise its bed in the upper parts. 



Chapter the 10th relates to the increase and diminution of 

 rivers, and the proportions in which they take place. Every 

 river is subject to variations in the volume of its waters and in 

 the capacity of its bed, from natural and artificial causes. 



It is also affected by winds and tides. An affluent which 

 enters into a river when its waters are at the lowest state of 

 depression, will maintain a greater elevation of surface than 

 when the river is highest*. A small river may enter into 

 a larger one without augmenting the section of the latter. 

 This apparent paradox is founded on the augmentation of 

 the velocity of the greater rivei', and Guglielmini quotes the 

 absorption of the Ferrara and Panaro branches of the Po by 

 that river, without any sensible augmentation of its channel: 

 this doctrine was first published by Castelli. The inutility of 

 diverting the waters of rivers by means of side cuts for the 

 purpose of lowering floods, is also insisted upon. 



Chapter the 11th relates to natural and artificial streams, and 

 the mode of conducting and distributing them for the purposes 

 of drainage and irrigation. In the former case the author con- 

 cludes, from a variety of reasons, that it is better to unite all the 

 waters of a region into one grand conduit, than to allow them 

 to run off by many separate conduits, and vice versa with respect 

 to irrigation. 



• The truth of this observation seems to be generally allowed, Mthough not 

 ■-satisfactorily established, — That the water rushes quicker down rivers in their 

 high than in their low state. 



2 F 2 



