FLOATIXG. 397 



riverside sawmill timber-depots, which will be described further 

 on. 



By supplj'ing booms with sluice-gates, they may be consider- 

 ably improved ; but this necessarily pre-supposes sufficient 

 strength to withstand the pressure of the wood and water. 

 Sluice-gates are specially valuable in the case of large booms 

 with masonry supports. By regulating the supply of water, the 

 front of the boom may be more uniformly covered with floating 

 wood, so that when the sluices are opened the greater part of it 

 may become stranded, or can easily be brought to land. In the 

 case of long booms, it is highly advantageous by opening first 

 one sluice-gate and then another, to drive the Avood in front of 

 portions of the boom hitherto free from it ; and, finally, by open- 

 ing all the sluice-gates to bring in the tail of the sweep of the 

 wood. 



iii. Reduction of the Pressure on a Boom. 



Attempts should be made in every possible way to reduce the 

 pressure on a boom, and this object may be secured in various 

 ways by constructing booms on weirs, by means of channels for 

 waste water, channels to remove sand, sluice-gates, &c. 



Lateral booms are generally placed on a weir, which supports 

 part of the water-pressure and reduces the fall of the stream and 

 the pressure on the boom. Nearly all large booms which are 

 intended to strand the wood, or to serve as lateral booms, are of 

 this nature. Channels for waste water are artificial cuts which 

 branch-off from the main stream above the boom, returning into 

 the stream below it. A certain portion of the water is thus led 

 away from the boom, which has, therefore, less pressure to with- 

 stand. Fig. 238 represents such a channel, which is supplied 

 at its outlet (ni) with a lateral boom and sluice-gates and sub- 

 divides below into several branches {b, b, b). If the terminal 

 boom were also in a side-channel, which has besides the advan- 

 tage of a weak current, its utility may be further increased by 

 smaller channels taken from above the boom and returning into 

 the other below it. 



Booms in streams which bring down boulders and gravel have, 

 besides the force of the current and of the floating wood, to with- 

 stand the pressure of the sand and boulders. Wherever, there- 



