38C ON EMBANKMENTS, PIERS, AND HARBOURS. 



the bottom; and it is at this point that the greatest strength is 

 required. But if the boards forming the sluice be fixed across it, 

 in horizontal directions, their strength must be greatest at the 

 bottom. 



In the construction of flood-gates, the principles of carpentry 

 must be applied in a manner nearly similar to that which serves for 

 the determination of the best forms of roofs. The flood-gates, if 

 they are double, without a solid obstacle between them, must meet 

 at an angle : and when this angle is very open, the thrust against 

 the walls or hinges must necessarily be very great. If, however, 

 the angle were too acute, the flood-gates would require to be 

 lengthened, and in this case their strength would be far more 

 diminishe'd than that of a roof similarly elevated, since the hydro, 

 static pressure acts always with full force in a perpendicular direc- 

 tion. The thickness required for each flood-gate may be deter, 

 mined in the same manner as the thickness of a sluice. 



Where a sluice-board of considerable dimensions is to be occa- 

 sionally raised, it may be necessary to ascertain the force which 

 will be required for overcoming its friction ; this friction is nearly 

 proportional to the whole pressure of the water, and may be found, 

 with surlicient accuracy, in pounds, by multiplying the square of the 

 depth of the sluice, in feet, by ten. Thus, if the depth be three 

 feet, the friction or adhesion will be about 90 pounds for each foot 

 of the breadth. 



If the side of a canal gives way, it is sometimes of consequence 

 to prevent, as much as possible, the escape of the water. For this 

 purpose it is usual to have doors or valves in various parts of the 

 canal, which, when the water is at rest, lie nearly flat at the bot- 

 tom ; but when it begins to run over them, with a considerable 

 velocity, they are raised by its force, and put a stop to its motion. 



The utility of the introduction of canals into a commercial 

 country may be estimated in some measure by the effect of the 

 same labour, employed in removing weights by land carriage and 

 by water. Thus* a single horse can scarcely draw more than a ton 

 weight on the. best road, but on a canal, the same horse can draw 

 a boat of 30 tons at the same rate. 



The construction of piers and quays, and the management of 

 harbours, are also important departments of hydraulic architecture; 

 it often happens that besides the application of the general princi- 



