Iurington.] DREDGING. 177 
charged about 10 to 15 feet astern of the dredge. On the double and 
single lift dredges no stackers were necessary, because the larger per- 
forations of the trommels allowed the comparatively small percentage 
of- stones to be discharged over the side without shoaling the water 
too much. 
The gold-saving apparatus introduced with the New Zealand type 
was also essentially different. The hydraulic mining undercurrent, 
which is a wide sluice with steep grade, and which spreads the material 
over a broad surface in a shallow sheet after all stones of diameter 
greater than 2 inches have been removed by a grizzly, was evolved 
in California but had been successfully used in New Zealand in hydrau- 
lic operations. A very slightly modified form of this device was used 
on a number of the dredges, with the difference that cocoa matting 
instead of riffles charged with quicksilver was used to catch the gold 
(see PL XXIX, A, p. 162). 
The double and single lift gold dredges depended principally, and 
in some cases entirely, on a comparatively narrow sluice with riffles 
charged with quicksilver. The material passed over the riffles in a 
deep sheet, a condition not conducive to a high percentage of saving, 
where the gold is in the form of very finely divided particles. 
"There is a. marked difference in the manipulation of the several 
types of dredges. The New Zealand boat in digging is held to the 
face of the bank by a headline run out over the bow to an anchorage, 
and the buckets are lowered in a vertical plane, the material being- 
caved by undermining at the bottom. The other types are held to 
the face by a pivotal stern spud (see PI. XXXI, J.), the buckets being- 
side fed horizontally through an arc of 120°. The cut is begun at the 
surface and the digging ladder lowered about 1 foot at the comple- 
tion of each arc, no attempt being made to undermine or cave the 
material excavated. 
Several shovel dredges were built at Oroville at about the time the 
New Zealand type was installed. On these the hydraulic-mining 
undercurrent, almost the same as that of the hydraulic miner in Cali- 
fornia, and a shaking screen, similar in action to the rocker of the 
early-da} T California miner, are used. Belt stackers were also 
installed. The shovel dredges proved costly of operation and have, 
with few exceptions, been discarded. 
Next in order of development came a dredge with the bucket 
evolved in canal and harbor dredging. The machine was very much 
strengthened, however, and was provided with shaking screen, belt 
stacker, and hydraulic-mining undercurrents, called " gold-saving 
tables." In this type is found a combination of the belter features of 
the double and single lift and shovel gold dredges. Small buckets 
were first used, but the tendenc} 7 had been toward increased capacity. 
This dredge was an improvement, but it had a fault common to all the 
Bull. 263—05 12 
