ITKlNGTON.j DK EDGING. 181 
The trommel (sec PI. XXIX. J, p. 162) is a better mechanism, 
requires less power for operation, and gives much less trouble in 
maintenance than the shaking screen and does cleaner screening, par- 
ticularly when the material is at all indurated or tenacious. Trommels 
have almost without exception been too small, and all late installa- 
tions are larger. Amalgamation by impact is now accomplished by 
a special arrangement of the gold-saving apparatus, and distribution 
of water is no longer a difficult problem on dredges where trom- 
mels are used. One of the most recent installations by a company 
which has extensively employed both devices includes a trommel. At 
present in most cases the trommel is in high favor among Oroville 
operators. 
Where there is an overburden of any considerable depth the use of 
a device to blank the trommel or screen will be a distinct improve- 
ment. By its use all barren material can be stacked well back of dredge 
and usually at a more rapid rate than if handled in the ordinary way. 
Overburdens, which it is always necessary to dispose of as quickly and 
cheaply as possible, usually consist of a fine loam or sand which can 
be run but slowly over the gold-saving devices because of clogging 
when handled rapidly. A blanking device was used for a time at Oro- 
ville and worked well, but, because of bad design, required too much 
time in blanking and unblanking the screen. Its use, however, allowed 
the beginning of dredging operations, on interior ground, without the 
use of a sand pump and the immediate excavation to a depth of 30 
feet, with a consequent minimum wasting of dredging ground. The 
use of such a device involves a large discharge of material from screen 
or trommel to the stacker where a troughed belt is used. The incline 
of the stacker must not be so steep as to permit the material, which is 
often a thin mud, to run back to the end. 
The use of a device to blank the hopper which shunts the material 
into the well has been tried, but has not been entirely successful. In 
this case also, bad design rather than bad principle was responsible 
for failure. The disposition of an overburden by this method is to be 
heartily recommended, especially where dredges work on headlines, as 
they are thus enabled to distribute the barren material across most of 
the width of the cut. 
The gold-saving apparatus, after many experimental modifications, 
including the introduction of various magnetic and intricate devices, 
has returned to the hydraulic-undercurrent type with the mercury 
riffle. For saving clean gold the mercury riffle is probably best. 
When, however, for &ny reason there is imperfect amalgamation, the 
use of some form of woven material to enmesh particles of rusty gold 
is good practice. 
The increase of the area of gold-saving surface has met with good 
results. Steeper grades are now used for gold-saving tables than for 
