4.'J6 FOURTH REPORT — 1834. 



Chapter the 1 2th treats of canals, and the precautions neces- 

 sary to supply them with water from rivers and reservoirs, such 

 as diminishing the force of the waters at their junction with the 

 canal, fortifying the points of junction, &c. The effects of sluices, 

 dams, regulators, aqueducts, siphons, and locks are all spoken 

 of in detail. 



Chapter the 13th relates to the drainage and warping of 

 marshes. The first principle is to intercept and prevent the ac- 

 cumulation of water, by diverting it from the borders of the 

 marsh, so that by the cessation of the cause, the effect will also 

 cease. In this manner the whole of Lombardy was drained*. 



The other principle is by raising the general surface of the 

 soil ; by allowing the water to deposit its earthy materials in 

 times of floods : this was called colmata, or warping, a practice 

 often adopted in Italy, where the rivers have not been allowed to 

 raise their beds to an unnatural height above the general surface 

 of the adjacent country by means of embankments. 



Chapter the 14th and last, is very important in an engineer- 

 ing point of view, as it treats of the effects of regulating and 

 shortening the courses of rivers. This operation oxight never 

 to be undertaken without a perfect knowledge of the soil through 

 which it is proposed to carry the river. Cuts and shortening 

 rivers with gravelly bottoms are rarely attended with success, 

 but where the soil is muddy or sandy, such works are more du- 

 rable. The author adduces the Po, which has established itself 

 in the middle of its basin, as an example of the equilibrium 

 which its course has attained by the rivers which flow into it on 

 both sides. The work of Guglielmini contains much valuable 

 information, although, from its numerous contradictions and 

 errors, particularly on the formation and transportation of 

 stones and gravel, it requires to be consulted with caution. 



The next author on rivers is Zanotti : this writer endeavours 

 to determine, by a series of observations, the position which the 

 beds of rivers should occupy near the sea, in proportion to the 

 superficies of their waters. 



In considering the sections of the Po and Tiber, he was of 

 opinion that the acceleration of the waters occasioned by the 

 freeness of the outlet in these rivers, extended up the river to a 

 considerable distance, and reached to the spot which would be 

 struck by a horizontal line drawn from low-water mark. Finally, 

 on comparing these observations together in detail, he disco- 



* This was always the principle adopted by the late Mr. Rennie in draining 

 the fens of Lincolnshire and Cambridgeshire, by means of catchwater drains at 

 the bases of the surrounding hills ; and by uniting the scattered waters by large 

 drains, they were conveyed to the sea. 



