390 



WATEll-TKANSPORT. 



grating rails are very large and weigh several liundredweigbts 

 each, but even then, part of the grating must be removed if the 

 stream is to remain navigable, or i)assable 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. 



"Water-sawmills always require booms to keep out the float- 

 ing wood which is intended to pass beyond them. Such booms 

 must be constructed so that part of the grating may be readily 

 removable and allow entrance for the butts which are to be sawn. 

 The grating is, therefore, frequently provided with the arrange- 

 ment shown in fig. 231. The hooks at {n n) are for the removal 



Fig. 232. 



of the rails, each of which is perforated for the admission of a 

 wedge to keep it when raised in position, 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 chiefly used 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. The}- are 



