FLOATING. 



389 



the admission of a wedge to keep it in position when raised, 

 the wedges resting on the bar (a a). 



Besides the above usual kinds of booms, special, local booms, 

 such as trestle-booms, portable booms and booms with gabions, 

 are in use, of a cheaper and simpler mode of construction. 

 They are used chiefly for temporary floating, or in the case of 

 streams subject to such high floods that the construction of 

 more elaborate and expensive booms is not advisable. They 



Kijj. '2(\:>>. Moveuble support for booms. 



are therefore re-made every floating season, and then broken- 

 up, and are prevalent chiefly on the south side of the Alps, in 

 Savoy, the South Tyrol, Garinthia and other districts. 



The essential feature of a trestle-boom is a three-legged 

 trestle (Fig. 260). These trestles, strengthened by the trans- 

 verse pieces (a a), are placed in a line across the stream so 

 that one foot of each projects somewhat over the foot of the 

 trestle next to it, and the tops of all the trestles are about the 

 same height above water-level. Thus different-sized trestles 

 are required according to the depth of the water. In the case 

 of large trestle-booms over strong streams, a second row of 



