GRAIN-DUST EXPLOSIONS. 7 
ing down from the stock bin and elevator. The removable hopper 
is partially withdrawn from under the mill platform. Above this 
hopper is one of the steel carbon holders entering the wooden door 
which has replaced the iron door of the lower part of the hood. 
In this wooden door is located a peephole, covered with mica, for the 
purpose of observing any sparks or flashes that may occur within 
the hood. On the opposite side of the mill (PL II, fig. 2) is a similar 
arrangement of peephole and carbon holder. The rheostat and 
ammeter for regulating the carbon arc, located on the edge of the 
hopper of the floor dump, are shown on the right. On the floor back 
of these instruments can be seen the recently installed relief pipe (tv), 
leading from the hopper to the outside of the building, a distance of 
8i feet. 
Plate III, figure 1, shows the dust room, located at the end of the 
conveyor, and the small door which gives access to the basement of 
the mill building. A small box (E) was built over the open end of the 
conveyor for the purpose of inclosing the carbon arc used in Series 4 
(p. 14.) A relief pipe extends from this box up through the roof, 
and is capped by the hood (F). 
Figure 1 gives a sectional view of the 16-inch attrition mill 
which was used for the tests described in this report. The grinding 
Fig. 1. — Sectional view of ball-bearing' attrition mill. 
plates or disks (d x and d 2 ) revolve in opposite directions at about 
2,200 revolutions per minute. The upper part of the hood or casing 
(c) inclosing the grinding disks can be readily removed. H is the 
feeding hopper where the grain is introduced and fed by a "wabbler," 
which is run by the friction pinion on the shaft (f ) . Horizontal slides 
within the hopper regulate the amount of feed. The adjustment of 
the fineness of grinding is accomplished by hand wheels (1 and 2), 
the latter serving as a locking device. The tension spring (ts) holds 
