2G0 On the Cunsiruction of Dams, 



braces of the frame, stout planking should be spiked over 

 these logs, so as to present a smooth surface for the waste 

 water to flow over, and to act as a protection to the stones, 

 earth, &c. below. The front of the dam should also be 

 filled in with earth, rubbish, &c. ; and if the surface of the 

 rock is so uneven as to prevent the front logs bearing fairly 

 upon it, brushwood and fascines may be placed in front, so 

 as in some measure to close the spaces between the rocks 

 and the logs.* 



When the bed of the river is composed of sand, clay, or 

 material too soft to resist for any length of time the action 

 of the water, the plan shown in figure 2 may be advan- 

 tageously adopted. This frame is composed of a sill, 

 extending not only the width of the dam, but also of the 

 apron in rear, notched down and pinned to three or more 

 sleepers, which are laid transversely to the stream, and sunk 

 into the bed of the river. Into this sill, the beam {a h) is 

 framed at an angle of about 30° with the horizon, and sup- 

 ported in this position by the two struts {he) {de) at an angle 

 of about C0°. These frames are placed at about 8 feet apart, 

 and upon them are notched the horizontal beams which 

 carry the planking with which both the up and down stream 

 sides of the dam are covered. 



In order to prevent the water making its way under the 

 dam, a row of plank piling {d), about 5 feet long and 4 

 inches thick, should be driven in front of the upper sleeper, 

 and a line of waling (/) upon this row of piling should be 

 well spiked through the piles into the frame. In order to 

 secure the work more completely against leakage, clay should 

 be thrown in front of the sheet piling to a height of one or 

 two feet. An apron (A,) as shown in figures 2, 3, is a neces- 



* Occasionally, when the strata cross the bed, and particularly when they 

 crop out against the stream, great additional stability may be obtained by 

 abutting the lower parts of the dam against the basset edges of the rock. 



