186 
GRAVEL AND PLACER MINING IN ALASKA. 
[bull. 263. 
the constant danger of the dredge not being located with the pivotal 
point in the right place and thus, when changing from one half cut to 
the other, there is a consequent loss of ground. In this figure it is 
assumed that the dredge is digging %) feet below the surface of a 
pond on a bank 30 feet deep. 
In fig. 39 the result of keeping the lines of advance of the pivotal 
points too close together is shown. The dumping of tailings at x' and 
x would result in grounding the dredge. In this case the distances 
from z and z' to sides of cut are unnecessarily great. It is evident 
that if the lines of advance of the pivotal points are kept too far apart 
ground adjacent to the cut will be wasted. In general, it may be said 
that where the material to be dredged is free, the headline method is 
Fig. 38. — Correct spud digging from two pivotal spuds. 
superior, and also, if by the introduction of some device, surging can 
be prevented, the headline method would be superior in dredging 
indurated material, for there the problem of the disposition of the tail- 
ings is much simpler. This permits of the use of shorter stacker and 
tail sluices and, therefore, smaller hulls and lighter construction. 
There is a marked tendency now toward stronger hull construction, 
and longitudinal bulkheads or fore and aft keelsons are often intro- 
duced. Traverse water-tight bulkheads at the bows would be advan- 
tageous in dredges working in torrential rivers where there is danger 
of collision with floating trees; or, in rivers like those in Alaska, where 
ice may be encountered. 
In some instances machinery too heavy for the size of the hull has 
been installed, and, as a result, there has been a lack of stability of the 
