FLOATIXG. 



387 



supports arc wanting, and the pressure of the sweeps of timber 

 is great, masonry-pillars must be erected for the purpose 

 (fig. 228). 



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 frequently 

 placed at the water-level (fig. 226) where it is best preserved. 



In the case of large booms required to withstand the pressure 



Fig. 228. 



~^c=. 



of large sweeps of floating wood and powerful streams, oblique 

 gratings are used. It is evident that such an arrangement will 

 withstand a much greater pressure than vertical gratings. The 

 inclination of the grating to the surface of the water varies, 

 depending on the absolute weight and stability of the rails which 

 form the grating. Where these rails are large — in large booms 

 they often attain lengths of 20 — 25 feet and a thickness below of 

 8 to 10 inches — the inclination of the grating may be 60°, but 

 otherwise it is placed more obliquely, say, at an angle of 25° 

 to 30°. 



The rails of the grating are always round pieces, barked spruce 

 or larch, with their thicker ends in the water, and they rest 

 without any support on the bed of the stream. In front of them, 



c c 2 



