312 LECTURE XXVI. 



The art of embankment is a branch of architecture entirely dependent on 

 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,ithas been 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 im- 

 portant subjects have been discussed by various writers, in many copious 

 treatises, expressly devoted to hydraulic architecture. 



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 overturn 

 them; by their laterval adhesion, the force tending to thrust them aside hori- 

 zontally; 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 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 l^ss than a right angle. The pressure acting on a bank thus 

 inclined will also tend to condense the materials, and to increase their la- 

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

 their weight, than if the 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 effectually. An embankment of this 

 kind is usually furnished with drains, formed by wooden pipes or by brick- 

 work,, 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. (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 



