438 LOGGING 
On account of the large amount of water they must carry to 
float logs and because of the wear-and-tear they receive, the 
boxes are made of strong material supported on cribwork which 
is kept as near the ground as is feasible. 
Sluice boxes are sometimes made with two thicknesses of 2-inch 
plank, the inner set being surfaced and tongued and grooved to 
insure a tight joint, while the outer planks break joints with 
the inner and make a tight box. A sluice of this character built 
in the Lake States for white pine was 36 inches wide at the base, 
108 inches wide across the top, and 60 inches high. The water 
in the sluice was controlled by half -moon gates (Fig. 136), located 
at the mouth of storage reservoirs. 
TRESTLES 
Trestles may be built of round timber or of 2- by 6-inch or 
4- by 8-inch sawed material. Flumes used for transporting sawed 
products usually have a trestle made from square-edged material, 
because it can be secured at the mill and transported to the place 
of construction in the completed portion of the flume. Where 
logs, pulpwood, acid wood, and other rough material are trans- 
ported from the forest to the manufacturing plant, round timber 
from 8 to 12 inches in diameter is often used for trestle construc- 
tion for it usually can be secured in the vicinity, although some 
prefer to erect a portable sawmill at the head of the flume and 
manufacture lumber for its construction. 
Caps for round timber trestles are made either from small 
timbers hewed on opposite faces to the desired thickness or from 
sawed material. Stringers are usually made from sawed timber. 
The braces for round timber trestles are made from small poles. 
Caps for square-edged timber trestles are made from 2- by 6-, 
4- by 4-, or 4- by 6-inch material, and stringers from 4- by 4-, 4- 
by 6-, or 6- by 6-inch timbers, the choice depending on the size 
of the box, the distance between trestle bents, and the amount of 
water carried. 
Braces for the box are placed along the stringers at 2-, 4-, or 
8-foot intervals, depending on the length of the span, the form of 
the box,^ and the strength required at special points, such as 
1 A V-box with a backbone for fluming lumber requires bracing only at 
8-foot intervals, while a box flume should have braces every 4 feet on a 24- 
foot span. Loading points on log flumes are often braced at 2-foot intervals. 
