FLOATING. 



387 



Baden, near Vienna, also that over the Ilz at Passau, the boom 

 nearly a kilometer (f mile) long at Brixlegg, and other large 

 booms. 



What enormous pressure such booms have to support, 

 especially in floods, may be imagined easily from the fact, 

 that floating timber often accumulates behind them to a 

 height of 15 to 20 feet and sometimes even overtops them. 

 In such cases, as has been already remarked, not only must 

 the construction of the boom be of the strongest possible 



Fi<:. L'i',0. Tit-stir to 



a L'i;uini:. 



character, but also the locality must be specially adapted 

 for it. 



In the case of many booms, with either vertical or oblique 

 gratings, the latter are placed in situ only during the floating 

 season, and for the rest of the year are removed and kept in 

 sheds on the river-banks. This cannot be done always, when 

 the grating rails are very large and weigh several hundred- 

 weights each, but even then, part of the grating must be 

 removed if the stream is to remain navigable, or passable by 

 rafts of wood. In such cases, the rails are provided with 

 strong iron rings so that they can be raised by means of 

 hooked poles and placed on the horizontal bars, and on a 

 planked footway constructed behind the latter. 



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