384 



WOOD TRANSPORT BY WATER. 



Booms may be divided into two classes, according as the 

 grating is vertical or oblique, the largest and most important 

 belonging to the latter category. 



Fig. 254 represents a simple form of wooden boom with a 

 vertical grating which has to resist a moderate pressure only ; 

 Fig. 255 shows the section of a support to this boom, and m 

 the grating and horizontal bars. Wherever in mountain- 

 streams rocks occur on which the grating may be supported, 

 they may be utilised as supporting piles for the boom ; but if 

 such natural supports are wanting and the pressure of the 



Fig. 256. Boom with stone supports. 



sweeps of timber is great, masonry-pillars must be erected for 

 the purpose (Fig. 256). 



The horizontal bars are constructed of large balks of 

 timber, which are bored through in order to allow the rails of 

 the grating to be inserted ; or they are composed of three 

 balks, the middle one perforated to support the grating. The 

 lower bar is placed frequently at the water-level (Fig. 254), 

 where it is best preserved. 



In the case of large booms required to withstand the 

 pressure of large sweeps of floating wood and powerful streams, 

 oblique gratings are used. It is evident that such an 



