500 ON EMBANKMENTS, P1EIIS, HARBOURS, 



to thrust them aside horizontally j and by their density and tena- 

 city, the penetration of the water into their substance. If the 

 water be in motion, they must also be able to resist its friction, 

 without being carried away by it, and they must be arranged in 

 such a form, as to be least liable to be undermined. For many of 

 these reasons, the surface of the bank exposed to the water must 

 be inclined to the horizon : the line expressing the general direction 

 of the pressure of the water ought to be confined entirely within 

 its substance, so that no force thus applied may be able to overturn 

 it as a whole ; and this condition will always be fulfilled, when the 

 sides of the bank make an angle with each other not less than a 

 right angle. The pressure acting on a bank thus inclined will also 

 tend to condense the materials, and to increase their lateral adhe- 

 sion, and the particles will become less liable to crumble away by 

 their weight, than if I he surface were more nearly vertical. For 

 embankments opposed to the sea, a bank much inclined has also 

 the additional advantage of breaking the force of the waves very 

 eifectually. An embankment of this kind is usually furnished with 

 drains, formed by wooden pipes or by brickwork, closed by falling 

 doors, or valves, which allow the water to flow out at low water, 

 but do not permit the tide to enter. To prevent the penetration of 

 the water, clay is often used, either mixed with gravel, or sunk in 

 a deep trench cut on each side of the canal or reservoir. 



The greater or less velocity of a river must determine what sub. 

 stances are capable of withstanding its tendency to disturb them ; 

 some are carried away by a velocity of a few inches in a second, 

 others remain at rest when the velocity amounts to several feet. 

 But in general, the velocity of a river is sufficient to produce a 

 gradual transfer of the particles of its bed, which are shifted slowly 

 downwards, towards the sea, being occasionally deposited in those 

 parts where the water has least motion, and sen ing at last to form 

 the new land, which is always advancing into the sea, on each side 

 of the mouth of a large river. It has been recommended, as a 

 good form for a navigable river or canal, to make the breadth of 

 the horizontal bottom one-fifth of that of the surface, and the depth 

 three-tenths. 



If a canal or a reservoir were confined by a perpendicular surface 

 of boards, and it were required to support it by a single prop, the 

 prop should be placed at the distance of one-third of the whole. 



