278 ANNUAL REPORT 
with the shaft, thus giving the mill two direct actions on the material to 
be ground. A pressure of 6,000 pounds is brought to bear on the 
material to be pulverized between the roll and the die. 
When a quantity of the material is fed into the mill, filling the 
pan as high as the shoes, on the lower side of the roll, they stir it up 
and throw it against the ring so that it is acted upon by the roll and 
crushed. All that is sufficiently fine passes at once through the screen, 
the coarser portion falling down to be acted upon again. The fan at- 
tached to the shaft above the roll draws air in at the top of the cone, 
forcing it through the screens and out into the discharge. A 16-mesh 
screen delivers a product of which over 90 per cent. will pass a 
6o0-mesh screen. 
The roll weighs about 100 pounds and has a diameter of 18 
inches. The mill makes 200 revolutions per minute and produces about 
3,500 pounds of ground clinker per hour and probably 5,000 pounds of 
raw stock, consuming 25 to 27 horsepower. Mechanically the machine 
is very efficient in its action, but being unbalanced, it requires a heavy 
foundation. Its drawbacks are: 
1. A tendency to segregation of the raw stock owing to its centri- 
fugal action. 
2. Heavy repair expenses. 
Of two particles of the same specific gravity, but of different 
diameters subjected to centrifugal force the larger one will move toward 
the circumference more rapidly ; of two particles of different specific grav- 
ity, but of the same diameter, the heavier one will also move outward more 
rapidly. On the other hand, in a current of air of uniform velocity the 
larger and heavier particles “will not be carried as far as the smaller and 
lighter ones. If now in the Griffin mill the force tending to separate the 
particles by throwing the larger particles farther outward were equal 
to the force produced by the fan, which carries the finer particles farther, 
no segregation would take place. Just in how far these forces are 
balanced in this mill the writer does not know, but it seems that the 
centrifugal force predominates. The fans also give motion to the 
particles by impact. It may be that this difference is of no practical 
importance, but it must stand as being reasonable until contradicted 
by definite data. It appears, hence, that in a mixture of limestone and 
clay the lighter clay would be blown away from the stone, while in 
a mixture of cement rock and limestone of about the same specific 
eravity no appreciable separation should take place. It must be remem- 
bered that a further separating action holds true in the space beneath 
the pan when the particles drop by gravity into the receiving trough. 
In this respect the action of the tube-mill seems to be superior to that 
