.S90 



WOOD TRANSPORT BY WATER. 



trestles is placed behind the first to strengthen it, one of the 

 feet of the second row crossing the feet of those in the front 

 row. This crossing of the feet of the trestles strengthens the 

 boom in a very marked way. 



After all the trestles are in position in the water, the bars 

 ()> l> />) are nailed on to them ; they are intended to support 

 heavy logs with which the trestles are loaded, to add weight 

 to the boom and render it firmer. As the trestles are not 

 imbedded in the ground, but only rest on it, they would 

 not withstand the force of the stream if they were not 

 heavily weighted. Further weight is added by placing stones 



Fig. 264. Gabion boom. 



and boulders above the logs which rest on b b b. Supports 

 for the rails are then nailed on to the trestles, the rails 

 fastened to them with withes, and floating logs placed in front 

 of the rails. Fig. 262 represents a form of trestle-boom 

 common in S. Alpine countries. 



Portable booms form another class which may be erected 

 and removed at pleasure, but their mode of construction varies 

 considerably. Fig. 263 represents a section of such a boom 

 with a permanent base, which is used in streams where sudden 

 floods occur, as in Lower Austria, the rivers Ziller, Gail, etc. 

 The fixed base is composed of a beam (a) and piles (c c) ; on 

 the latter the trestle-beams (m w) rest, and the grating-rails 

 (d d) are supported by pieces (b b) which are bolted to tu. 

 Another kind of portable boom is used in Nadworna, in 



