238 LECTURE XXVI. 



hydrostatical and hydraulic principles. In Holland and in some parts of 

 Germany, this art is indispensable to the existence of large tracts of 

 country ; and even in this island it has heen of extensive utility, in gaining 

 and securing ground on the sea coast. The construction of canals, and 

 the management of rivers and harbours, are also dependent on the -same 

 principles ; and these important subjects have been discussed by various 

 writers, in many copious treatises, expressly devoted to hydraulic archi- 

 tecture. 



When a bank or dike is to be constructed, it must be composed of ma- 

 terials capable of resisting, by their weight, the effort of the fluid to over- 

 turn them ; by their lateral adhesion, the force tending to thrust them aside 

 horizontally ; and by their density and tenacity, 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 sub- 

 stance, 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 righl^angle. The pressure acting 

 on a bank thus inclined will also tend to condense the materials, and to 

 increase their lateral adhesion, and the particles will become less liable to 

 crumble away by their weight, than if the surface were more nearly ver- 

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

 the additional advantage of breaking the force of the waves very effectually. 

 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 re- 

 servoir. (Plate XXI. Fig. 281.) 



The greater or less velocity of a river must determine what substances 

 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 serving 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. (Plate XXI. 

 Fig. 282.) 



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, as we have already seen, at the distance of one third of the whole 

 height from the bottom ; but it would be always more convenient in prac- 



