Drainage by Gravitation, 1 5 



3| inches per mile for the first 1 2 miles above the outfall. The 

 surface inclination of the canalised part of the river Witham 

 is about 3J inches per mile. The tidal portion of the river 

 Ouse between the upper end of the Eau Brink Cut and 

 Denver Sluice, which has a very irregular section, has a mean 

 fall in floods of 1 1 inches per mile, the greatest fall being at 

 the rate of 17 inches for three-quarters of a mile round a sharp 

 bend in the river, falling to 3 inches per mile through the Eau 

 Brink Cut to Lynn. The inclination of the Shannon, where it 

 passes through flat land, is at the rate of 2\ inches per mile. 

 In the main drains and canals in the polders in Holland 

 the surface inclination varies from ij to 3 inches in a mile. 



Slope of Side of Drain. — The determination of the 

 slope, or batter, to be given to the sides must depend on the 

 nature of the soil, but will also be guided to a certain extent 

 by the width and depth of the drain. It is frequently advan- 

 tageous to lay out the slopes of the main outfall drains at a 

 flatter batter than the nature of the soil absolutely requires, in 

 order to afford larger storage room during the time the flow 

 of the water is stopped by the tide, the rapid increase in the 

 area above the mean water line affording a large resei-voir for 

 the water, without increasing the width at the bottom. On 

 the other hand, to make the slopes more than necessary 

 entails useless expense. Soils of a sandy nature will require 

 very flat slopes, as the soil is easily removed by the wash of 

 the water. In the alluvial soils of marsh districts the large 

 drains may frequently be found having batters of only one to 

 one. Clay, according to its nature, will stand with a small 

 slope, so far as the wash is concerned, but i^ liable to slip if 

 laid too steep. Peat requires very little batter, in fact, many 

 of the secondary drains in the peat districts may be seen with 

 their sides almost vertical. A careful observation of the 

 existing drains or watercourses in the neighbourhood, and the 

 slope to which they have adapted themselves, will form a guide 

 as to the section to be given to a new drain. 



Main drains seldom have less slopes than 2 to i, and this 



