Modern Impwements in Coi-n-Milling Machinery. 105 
Appliances are fitted to all roller-mills by which an elastic 
pressure may be applied to the material passing between the 
rollers. The smooth rollers accomplish their work more by a 
crushing than a cutting action, and consequently greater pres- 
sure is applied through them to the goods than is the case with 
grooved rollers. And as the pressure is in proportion to the 
length of the working surfaces, the greater the length the 
greater is the pressure on the axles, and the greater the diflS- 
culty in keeping them cool and the bearings free from wear, 
so that of necessity smooth rollers are shorter than grooved 
rollers. The surface of these rollers, though smooth, is not 
polished ; the condition desired is that the surface of the metal 
should be quite smooth to the touch, while it has a dull appear- 
ance which has been likened to finely frosted glass. It is this 
very slight roughness of surface which gives the rollers a slight 
bite upon the material that passes between them, and thus, by 
means of pressure and the difierence of speed at which the sur- 
faces of the two rollers are made to travel, enables them to 
resolve it into flour. It is because porcelain has a very slight 
granular surface that some pi'efer to use smooth rollers of this 
material. 
A " roller-mill " consists of an iron frame of very solid con- 
struction, containing generally one or two pairs of rollers, and is 
fitted with all necessary appliances. Roller-mills have been built 
containing as many as six pairs of rollers, but this construction 
has been abandoned. In the ordinary type a pair of rollers is 
placed horizontally in the frame, with their spindles in the same 
plane ; the carriages or bearings of the spindle of one roller are 
fixed to the frame ; the bearings of the second roller are movable 
and adjustable, so that the second roller can be made to approach 
or recede from the first roller, but always in perfect parallelism 
with it. To apply pressure to the material to be reduced by 
passing between the rollers various devices are adopted, em- 
ploying either alone or in combination screws, wedges, levers 
and weights and springs. But while it is important that the 
pressure should be constant in amount, it is also necessary that 
it should not be rigid, in order that provision may be made for 
the recession of the movable roller if a piece of metal or other 
ungrindable object should pass between the rollers. There 
must be power to exercise the most delicate adjustment from 
time to time to suit the work, and the pressure must be con- 
stant and alike at each end of the rollers, and there must be an 
arrangement for throwing the rollers instantly apart in case of 
need, and of bringing them to the original relative situation 
without entailing the trouble of readjustment. 
Placed above the rollers is a hopper, containing the material 
