176 GRAVEL AND PLACER MINING IN ALASKA. [bull. 263. 
The particular problem at Bannock was to evolve a dredging pump 
to stand the wear and tear of handling the fines of about 2,000 cubic 
yards every twenty -four hours. The first pump was worn out after 
seventy hours' use, but another of excellent construction, designed by 
the dredge manager, was installed and used for several years. After 
a seven months' run, when the fines of over 400,000 cubic yards had 
passed through it, it was estimated that the costs of repairs and 
renewals of the pump had been only* about $150. 
With a view of eliminating the dredging pump the so-called single- 
lift dredge was designed by Montana men. This type discharges the 
material about 28 feet above the deck into a trommel with perforations 
5 inches in diameter. As in the "double-lift" dredge, the coarse 
material was deposited over the side, while the fines dropped into a 
sluice and were discharged about 100 feet astern. The greater height 
of discharge of buckets made a secondary lift of the fines unnecessary. 
This form of construction introduced a new set of problems, as the 
higher position of the upper tumbler necessitated longer ladders, more 
and heavier buckets in a longer bucket chain, larger hulls, and heavier 
construction throughout. Some of the first u single-lift" dredges 
were very unstable because of the high position of center of gravity. 
This type of dredge is now in more general use than the " double-lift" 
machines. 
The dredge which had been used so successfully in New Zealand 
was not introduced into the United States until after the extensive 
operation of these two types. The first New Zealand dredge was 
installed in the Oroville district of California, and after a few modifi- 
cations to suit. local conditions was and is still operated successfully. 
This was followed by the building of a number of similar dredges in; 
California, Colorado, Idaho, Oregon, Montana, and elsewhere (see 
Pis. XXX, A, and XXXI, B). 
These dredges were essentially different in design and manipulation 
from those of the double and single lift types. While the bucket 
of the New Zealand t}^pe was evolved entirely for gold-dredging pur- 
poses, the bucket previously used in this country was but a slight 
modification of the one used for harbor and canal dredging. In both 
t} r pes, but more notably in the bucket evolved in harbor and canal 
dredging, the first made were deficient in the strength of the digging 
apparatus. 
The New Zealand t} r pe of gold dredge discharges the material about 
16 feet above the deck into a trommel, the largest perforations of 
which are about one-half inch in diameter. A stacker (see PI. 
XXXI, B) made of a chain of steel buckets elevates all material not 
passing through perforations of trommel and discharges it astern of 
the dredge at a height sufficient to allow for the expansion of the 
tailings. The fines pass over the gold-saving devices and are dis- 
