WORKS FOR CREATING SLACKWATER. 143 



For locks exposed to high floods, however, masonry should always be used, as its 

 mass is necessary for endurance. The choice of materials for it is limited usually to 

 sandstone, limestone, or concrete. Limestone is usually preferable to sandstone, as 

 it does not soften and wear so easily under the action of the water.* Concrete is very 

 largely used at the present time, although experience with its durability in river works 

 has been short. There seems to be no reason, however, why it should not prove 

 satisfactory. For works exposed to the action of a river it is best to use a concrete of 

 Portland cement, at least above water. The difference between the cost of Portland and 

 natural cements at the present day is small, and the former will give a harder and 

 better concrete in all particulars. The higher grades of natural cement have given 

 excellent results in foundations, but the cheap grades, of which there are many on the 

 market, have not proved satisfactory for important work. 



The use of timber for permanent construction, wherever it will be exposed to the 

 action of moving water, should never be permitted. The accepted belief that timber 

 below water lasts forever is only partly true, for in order to do so it must be protected 

 from all currents. This fact was discovered long ago by European engineers, but in 

 America we find many examples of wooden lock floors, wooden foundations, and other 

 vital parts which the water will sooner or later wear away. The sheathing of a dam 

 bears strong witness to the rapid wearing force of water; and while a lock floor is 

 exposed to much slighter currents, the effect is just as sure. We have seen wooden lift- 

 walls, exposed simply to leakage from the gates, become dangerously worn in twenty 

 years, and have removed timbers from the heart of a dam, where very little water 

 could have reached them, which were worn and channeled half away. Wherever it 

 can possibly be done, nothing but masonry should be exposed to the action of the water. 



Protection of Banks. This will be found described under "Fixed Dams (Abut- 

 ment)." 



Clearing the Pool. After the location has been selected, the river above should 

 be cleared of bowlders, snags, and timber throughout that part which will be affected 

 by the new pool. It is best to commence this work at once, so the debris will disappear 

 before construction is finished, and also so that steamboats may be able to utilize the 

 stream in moderate stages of water as soon as the obstructions have been removed. 



If it is not expected to complete the lock and dam for some years, the trees can 

 be merely deadened, or "ringed." When this is done, the trees will gradually die and 

 fall, a branch at a time, into the river, and will be safely carried out by the floods. 

 Some trees, depending on size and species, will die and begin to disappear in eighteen 

 months, while others, especially sycamores, will struggle for life from three to six years. 

 Where they are below the level of the new pool, they should be deadened close to the 

 roots, so no stump will be left when the tree falls. 



* The softer classes of limestone, especially the oolitic varieties, have not proved durable in the face- 

 work of lock masonry, as after a few years' exposure to the weather the stones begin to scale off. For 

 a similar reason no machine-finished : urface and no bush-hammered surface should be used in exposed 

 work, as the action of the tools deadens the surface and causes it to scale. 



