398 WOOD TRANSPORT BY WATER. 



Booms in streams that bring down boulders and gravel, 

 besides the force of the current and of the floating wood, have 

 to withstand the pressure of the sand and boulders. Wherever, 

 therefore, the fall of the stream is considerable, it is sufficient 

 usually to place the boom out of the main current by allowing 

 surplus water to pass off by a side-channel ; or, when the 

 boom is in a side-channel, a deep and steeply inclined cut, 

 termed a sand-canal, is made in the latter to carry the sand 

 and boulders into the main stream. Fig. 269 shows the float- 

 ing-channel (, s), which bifurcates from the main stream, 77,- 

 in, m form so many cuts between strong, solid masonry walls, 

 which may be closed by lateral booms and sluice-gates ; a is 

 the sand-canal, which at d is only half a meter deeper 

 than the rest of the floating channel, but deepens gradually 

 towards p. The boulders which accumulate in d, p are 

 passed through a temporary opening (p) and the corresponding 

 sluice-gate in the lateral boom, and pass along m into the 

 main stream 77. 



Simple sand-canals can be opened for the passage of silt, 

 etc., only whilst floating is not in progress. In order to free a 

 floating-channel from these accumulations during the floating, 

 they may, as at q, Fig. 269, be covered with a wooden 

 lattice-work (Fig. 270) constructed at the bottom of the float- 

 ing-channel (). Besides this method, double booms may be 

 used, which are erected the one close behind the other, and 

 the silt and boulders are admitted into the interval between 

 them by opening the first boom and then passed through the 

 second boom, so that always one of the booms is ready to stop 

 the floating wood. In order also to expose the bed of the 

 stream in front of a boom and then strand the timber, deep 

 culverts with sluice-gates may be made and opened to pass the 

 water under the boom. 



(c) Further Details regarding Booms. In the preceding 

 paragraphs, a distinction has been made between lateral or 

 terminal booms, but the latter maybe of several kinds. Every 

 boom, whatever its dimensions, which catches floating wood at 

 a timber-depot is a principal boom. Owing to certain condi- 

 tions of a locality and want of sufficient space, it is not always 

 possible to supply every river timber-depot with a principal 



