392 



W ATEll-TR A N S PO RT. 



but only rest on it, they would not withstand the force of the 

 stream if the trestles were not heavily weighted. Further weight 

 is added by placing stones and boulders above the logs which rest 

 on {b h h). Supports for the rails are then nailed on to the 

 trestles, and the rails fastened to them with withes, and floating 

 placed in front logs of the rails. 



Portable Booms form another class which may be erected and 

 removed at pleasure, but their mode of construction varies con- 

 siderably. Fig. 233 represents a section of such a boom with a 

 permanent base, which is used in streams where sudden Hoods 



occur, as in lower Austria, the rivers Ziller, Gail, I've. The fixed 

 base is composed of a beam {a) and piles (c c) ; on the latter the 

 trestle-beams {m m) rest, and the grating-rails {d d) are sup- 

 poited by pieces {h h) which are bolted to {m). Another kind of 

 portable boom is used in Nadworna in Galicia, in which three 

 twisted wire ropes are stretched as tightly as possible one above 

 the other, and supported by trestles at distances of 30 feet 

 apart. 



Another kind of boom is formed of gabions (fig. 234), as used 

 in Yeuezianiscli and other places. Here, instead of wooden or 

 stone pillars, gabions of basket-work filled with stones are used, 

 which support the horizontal bars and the grating-rails. The 

 gabions are placed in a line across the stream at distances of 5 

 to 15 meters (IG — 48 feet) apart, according to the strength of 

 the stream, and are tall enoufrh to be above the highest water 



