24 ANNUAL. REPORT 
machine is supported by two heavy, hollow trunnions or shafts at the 
ends, through which the material 1s fed and discharged at the opposite 
end. The pebbles are charged by means of a manhole. A screw feeds 
the material into the hollow shaft. A wing is also arranged just outside 
of the mill to prevent any pebbles from leaving the mill at the inlet. At 
the exit end a screen is attached to the opening to prevent any pebbles from 
leaving at that end. 
Just what the real grinding action is in a tube mill has not been 
understood clearly until recently, and the views in regard to the appar- 
ently simple operation of this mill have not been at all well defined. 
Récently,* however, the operation of the tube-mill was studied in great 
detail by means of experimental glass mills, and sections covered at the 
end with a wire screen. In this manner it was possible to study the 
motion of the pebbles as well as of the material being ground. One exper- 
imental mill was 3 feet in diameter inside and was tried with various speeds ~ 
of rotation. When at rest the pebbles showed a height of 450 mm. 
The mill was now rotated with a speed of 23% revolutions per minute, 
and it was found that the pebbles rolled down the side rather slowly. 
On increasing the speed the pebbles seemed to loosen, and they rose 
to a height of 600 mm. with 32 revolutions. The pebbles have no 
motion until they are carried up the side of the mill, when at a certain 
height they become loose and drop back to the bottom, describing a 
curvilinear path which can be noticed quite distinctly. The pebbles 
farther away from the surface also do not move with reference to 
the circumference up to a certain height, but their downward path is 
more difficult to recognize. On increasing the speed to 35 revolutions 
the content of the cylinder is loosened still more, so that the pebbles rise 
to a height of 650 mm.; the downward path becomes quite distinct, and 
the lower pebbles separate in distinct layers. The pebbles of one layer 
do not mix with pebbles from the other layers. On speeding up the 
mill still more the pebbles, at 55 revolutions, form a solid ring around 
the circumference, without any displacement of the balls. With a speed 
of 34 revolutions a hollow, kidney-like space was observed. 
When material is to be ground it assumes the same motion as the 
pebbles, distributing itself within the spaces so that it is densest where 
the pebbles are closest, and looser where the latter are loose. The 
kidney-shape space, therefore, is free from the material—no rubbing and 
rolling between pebbles and the charge takes place anywhere except at 
the point where the pebbles strike the surface on falling from the highest 
point. There the pebbles strike with great force and exert an action 
similar to that of a stamp mill. ‘The pebbles, climbing up the side of the 
mill, drop away from it as soon as the vertical component of the forces 
acting upon it is equal to the centrifugal force, or 
* HT. Fischer, Zeitschrift des Vereins Deutscher Ingenieure. No. 138, 1904. 
