MEANS OF COMMUNICATION. 47 
pose, some being either too shaliow, or having many small crooked windings, 
which retard the current and cause bars; and in others the current is so 
swift as to render navigation dangerous. In all such cases certain works 
are necessary to render the streams navigable. By cutting off the small 
windings, and giving a river a straight course, the current will be increased 
and the formation of bars prevented. When the channel of a river is 
obstructed by rocks they may be removed by blasting ; where it is generally 
too shallow to bear vessels, successive portions of it are dammed up in order 
to obtain sufficient depth, the vessels entering the successive reaches by 
means of locks. Canals are constructed for the purpose of inland naviga- 
tion, where no natural means are available, or to connect one stream with 
another. Some of these structures we will now proceed to consider in 
detail. 
A. Dams. 
When shallow streams are to be made navigable by means of dams, the 
latter are built ofa height sufficient to maintain the depth of water required 
for navigation, and allowing the surplus water to run over the top, on which 
account they are called overfall-dams. They are built of wood or stone. 
In building wooden overfall-dams, large beams of timber are first laid in 
several contiguous rows across the bottom of the stream, and are firmly 
settled into its bed. Upon these sills are laid successive rows of beams, 
breaking joints, and planed throughout on the horizontal faces so as to pre- 
vent leakage; they are pinned together with treenails both vertically and 
horizontally, and the interstices between the vertical faces are closely packed 
with clay and sod. The sides of the dam are sloped towards the top, the 
pressure sustained at the bottom being much greater than at top; that face 
which slopes against the current is called the breast ; the downward slope is 
ealled the apron. To protect the breast from being undermined, a double 
row of thick plank is driven into the bed of the river above the breast. For 
the protection of the apron, piles are driven, on which a hearth of thick plank 
is laid to receive the fall of water, or else a bed is made of rocks firmly 
packed between the piles. 
On pl. 11, jigs. 16, 17, and 18, is represented a wooden overfall-dam 
across the river Witogra in Russia. This river being large and rapid, a 
very wide base has been given to the dam. The mode of construction is 
different in some respects from that above described, as is seen in the cross- 
section (jig. 17). The base consists of piles and grillage; the breast, F, is 
made by driving piles of different lengths so as to form the required slope. 
across which the breast-sills are laid, on which are spiked heavy oak planks 
closely fitted. A bulkhead of timber is built into the bank on each side of 
the dam, to prevent the water from passing through. Below the dam 
strong piles are driven into the bed of the stream, and between them large 
stones are packed to receive the slack of the falling water, and prevent the 
undermining of the dam. 
The upper surface of the dam is sloped in the direction of the current; 
thick planks are spiked upon the dam-sills. and their joints caulked and 
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