196 
GRAVEL AND PLACER MINING IN ALASKA. 
[BULL. 263. 
to working tests. It is proved that holes of less than three-eighths inch diameter 
unduly limit the discharge through the plates, are liable to choke, and that there is 
no advantage whatever to be gained by their use. The discharge through seven- 
sixteenths inch holes is sufficient in amount, and the fine shingle particles which 
pass through with the sands and seldom exceed one-fourth inch in diameter are not 
troublesome in their size and quantity. In fact, many consider that they are of 
benefit in keeping lively the sands on the tables. With holes exceeding seven- 
sixteenths inch in diameter, the size and quantity of small shingle become exces- 
sive, and the water passes away so rapidly as to prevent material from being carried 
forward over the plates. 
The above remarks are of somewhat general application for the use of 
punched iron screens in any form of undercurrent where it is assumed 
that a portion of the gold, including any nuggets which may occur, 
Plan 
Pole 
riffles 
Punched 
iron plate 
%"h\o/es 
Pole riffles 
Punched 
iron plate 
3/ Q " holes 
Hungarian riffles (see fig. 14) 
Longitudinal section of both main sluice and undercurrent 
Section on line A -B 
Fig. 43.— Kulibinka sluice and undercurrent used in Siberia. 
has been previously saved. In northern deposits where dredges have 
been installed, as in the Klondike and on Stewart River, punched 
iron screens used in revolving trommels have large holes, up to 11 
inches, even when a tailings stacker is used, while the average size 
in Oroville dredging practice is three-eighths inch. 
A screen installed in one box of the main sluice of a plant on Ophir 
Creek, Seward Peninsula, consisted of a number of longitudinally dis- 
posed round iron rods, acting as a grizzly, fitted above the bottom 
of the box, the gold being saved on mats below. This box, which was 
placed at the end of a 120-foot sluice, was said to save much fine gold. 
When it is desirable to introduce the principle of the undercurrent, 
