263 On the Construction of Dams, 



that is, of a series of rough cases formed of whole timbers, 

 notched together at the crossings, as shown in figure 5. 



In framing a dam of this description, two logs are laid in 

 a direction transverse to the stream at the same distance 

 apart as is intended for the width of the dam ; upon these 

 cross logs are notched at distances of G or 8 feet : other 

 transverse timbers are notched upon these, and the dam is 

 carried up in this way until it arrives at its intended height. 

 Sometimes it may be advisable to divide the interior spaife 

 into smaller compartments, by introducing more transverse 

 timbers ; during this, very little impediment has been offered 

 to the stream which flows through the interval between the 

 logs.- * When the crib-work is complete, the spaces betv/een 

 the cribs are filled with stone if it can be procured ; or if 

 not, with fascines, earth, &c., and a mass of earth and 

 rubbish is thrown into the river in front of the dam, so that 

 by degrees a mass is accumulated sufficient to prevent 

 leakage. This work is carried on simultaneously from both 

 banks, and as the water is checked, so the stream rises above 

 the dam, rushing through the central space left for its 

 passage. The same process may be continued till the dam 

 is completely closed ; but as large quantities of earth, &c. 

 would be washed away in attempting to close the opening 

 between the logs in this centre bay, the best plan is to 

 prepare a frame to receive a sort of gate made of logs, which 

 can be dropped down from above, and which will close the 

 opening sufficiently to prevent much Avaste of material 

 taking place. | 



When timber is scarce, fascines and hurdles may be used 



* In executing this sort of work, tlie first logs float on the water, and are 

 gradually sunk by the increasing superstructure. 



f Experience has likewise shown, that when the water is deep and even 

 rapid, the front of the dam may in like manner be formed of portions of crib- 

 work, two bays in length, constructed ashore, dropped down into position and 

 arranged on the arc of a circle in plan, beginning from each flank, filling them 

 as soon as properly placed. This, as the body of the dam, must be assisted 

 and supported by slopes of clay, as in figs. 1 and 2. 



