FLOATING. 



391 



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

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

 the South Tyrol, Carinthia, and other districts. 



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

 (fig. 232). These trestles, strengthened by the transverse 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 difterent sized trestles are required accord- 

 ing to the depth of the water. In the case of large trestle- 

 booms over strong streams, a second row of trestles is placed 

 behind the first to strengthen it, one of the feet of the second 

 row crossing the feet of those in the front row. This crossing 

 of the feet of the trestles strengthens the boom in a very 

 marked way. 



After all the trestles are in position in the water, the bars {h h h) 

 are nailed on to them ; they are intended to support heavy logs 

 with which the trestles are loaded, to add. weight to the boom and 

 render it firmer. As the trestles are not imbedded in the ground, 



