394 WOOD TRANSPORT BY WATER. 



fall and where sudden floods are not to be feared. They have 

 to resist severe pressure, and wherever large sweeps of wood 

 are floated should be constructed strongly. 



Oblique booms are commoner both in the case of lateral 

 and terminal booms, they have naturally a greater length in 

 proportion to the breadth of the stream than straight booms, 

 and the longer they are, the better able are they to withstand 

 the pressure of the floating wood and floods. Most booms are 

 not straight, but have a broken line of contour ; and many 

 booms, and some of the most important ones, collect and 

 retain for some time a large quantity of the floating wood. 

 The boom across the river Ilz, near Passau, built as in Fig. 258, 

 and a plan of which is given in Fig. 266, collects over ten 

 thousand saw-mill butts, and allows for their being continually 

 removed by the underground channel (a). 



ii. Lateral Booms. 



These booms are intended to divert floating timber into a 

 side-channel, and are long and oblique. 



In powerful floating-channels, a terminal boom usually 

 cannot be laid across the main stream without danger of 

 being broken. In such cases, therefore, a side-channel is 

 diverted from the main stream and the sweep of timber con- 

 ducted into it, the main stream being barred by a boom. 

 Fig. 267 is a long lateral boom, only closed in the middle 

 by floating logs. H is the main stream ; s, the side-channel, 

 lower down in which the terminal boom is placed ; b is a weir 

 diverting water into s. As in this case the pressure of the 

 sweep of wood and of the stream is divided between two 

 booms, neither of them need be very strongly constructed. 

 This is the chief advantage of leading the floating wood into 

 a side-channel. Where a natural bifurcation of a river does 

 not exist, an artificial side-channel is constructed frequently 

 with advantage ; if, then, the lateral boom is supplied with a 

 strong weir, or, if possible, with a sluice-weir, the supply of 

 water to the side-channel may be regulated at will. On this 

 general principle are founded all the better kinds of riverside 

 sawmill timber-depots, which will be described further on. 



