396 



LOGGING 



also cause the lumber to leave it. The maximum degree of 

 curvature permissible increases with the grade but diminishes 

 rapidly as the grade falls under 3 per cent. The following 

 table ^ of minimum grades is considered safe. 



The most desirable grades for a straight flume are 3 per 

 cent or more. Grades up to 75 per cent may be employed on 

 short stretches, provided all sharp changes in elevation have 

 properly proportioned vertical curves. 



TYPE OF BOX 



There are two general types of flume and sluice boxes. One 

 is V-shaped and may have a "backbone"- which makes a box 

 6 or 8 inches wide at the base, with outwardly sloping sides. 

 The other is known as the box flume. 



The choice of type and size of box depend on the character 

 and size of material to be transported, the amount of water 

 available, and the ultimate use of the water itself. In some 

 instances when water from flumes is used for irrigation pur- 

 poses, the box is of larger size than is required for the sole pur- 

 pose of transporting forest products. 



Lumber and log flumes rest on skids on the ground or are 

 elevated on trestles. They sometimes pass through tunnels or 

 cuts although these are avoided whenever possible because of 

 the increased cost of construction. 



V-box. — This t}pe of box is commonly employed for lumber, 

 crossties, small dimension stock, small round mine timbers, 



' From Lumber Flumes, by Francis R. Steel, Bui. of the Harvard Forest Club, 

 Vol. I, 1911. 



2 A triangular strip fastened in the vertex of the flume box. 



