38G 



WATER-TRANSPORT. 



which stop the sweeps of timber, and the ^a-ating of rails which 

 surmounts the boom. 



Booms may be divided into two chisses according as the 



grating is vertical, or oblique, the largest and most important 

 belonging to the latter category. 



Fig. 226 represents a simple form of wooden boom with a 



Fia. 2^ 



vertical grating which has to resist a moderate pressure only ; 

 fig. 227 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 

 utilized as supporting piles for the boom ; but if such natural 



