FLOATING. 



389 



and are arranged with their longest sides parallel to the stream 

 so as to offer as little resistance to it as possible. Of a similar 

 construction is the large boom at Baden near Vienna, that over 

 the Ilz at Passau, the boom nearly a kilometer (| mile) long at 

 Brixlegg and other large booms. 



What enormous pressure such booms have to support, especially 

 in floods, may be easily imagined from the fact that floating 



Fig. 231. 



^;<:;.^^>^^ 



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 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 always be done, when the 



