STATE GEOLOGIST. DANS 
before feeding into the tube-mill. As a machine it is decidedly inferior 
to the preceding mills in regard to economy of output. 
Dry Pan.—The dry pan, or edge runner, consists of a revolving 
pan from 6 to 9 feet in diameter, on which run two or more heavy 
mullers. Around the circumference a space of about 12 inches width is 
filled: with perforated plates. ‘The grinding is done on the solid plate, 
and the crushed material is scraped on to the perforated plates by fixed 
scrapers. The oversize material is thrown back on to the center of the 
pan to be crushed again. This machine is well suited for the intermedi- 
ate grinding of dry shales and clays, with which a 9-foot pan may show 
as high a capacity as 100 tons per day, but it is less efficient with hard 
limestone or soft clays. For softer stones it would show a good output. 
The iron frame pan does not need much attention and repairs. The chief 
drawback of the machine is its comparative coarse output, as it cannot 
be worked with a fine delivery, owing to the danger of choking, and the 
dust and noise it makes. A 9 foot pan requires from 30 to 40 horse- 
power. : 
Arranged in the order of their general efficiency these intermediate 
grinding machines would range about in the following order: 1, Rolls; 
2 Nome mill; 3, Disintegrator; 4, Ball-mill; 5, ae pan. 
FINE GRINDING MACHINES. 
Of these we have principally two machines, the tube and the Griffin 
mill, to consider, the use of other machines, like the buhr or emery mill, 
having become practically obsolete in American practice. 
Fig. 48. Tube mill. 
Tube Mill. ——Phe tube mill consists essentially of an iron shell from 
22 to 16 feet long and from 4 to 5 feet in diameter, filled somewhat 
above the axis with flint pebbles, imported mostly from Greenland. The 
18—s. G. Bull. 3. 
