PORTLAND CEMENT CONCRETE ROADS. 35 
nions, placed below the center of gravity of the load, to which the 
lifting yoke attaches. During the lifting the box is prevented from 
turning over by a hook attached to the yoke. When the box is in 
position tc dump, the hook is released and the box turns over on the 
trunnions. If the trunnions are properly located very little "kick 
back" is noticeable and the load is rapidly discharged. The side- 
discharge box is provided with a false bottom, which slopes toward 
the front of the box, where discharge is effected by releasing a hinged 
door which usually makes up one-half of the front side of the box. 
The side- discharge box throws the material well to the front of the 
loading skip, but is somewhat slow of discharge and has a slight 
tendency to " kick back." The sloping bottom necessitates a larger 
box and also places the center of gravity of the load higher above the 
rail than otherwise. The bottom-dump box is discharged by releas- 
ing the 2 hinged doors which constitute the bottom of the box. 
This type of box discharges very rapidly and is practically free 
from any "kick back." Batch boxes may be loaded by means of 
measuring hoppers attached to the loading bins, but this arrangement 
is not necessary, as the box itself serves as a measuring device. The 
proper height to which the boxes are to be filled with each material 
may be marked by means of thin nailing strips or bolt heads. The 
loading plant should be designed so that 4 or more boxes can be 
loaded at the same time. 
Batch boxes are usually loaded from open bins or a loading tun- 
nel. In tunnel loading the industrial train is run under the stored 
material and loaded from overhead traps. (See Fig. 2, PL VI.) 
The tunnel may be partly or wholly excavated into the ground or 
it may be constructed of wood on the surface of the ground. The 
material in either case is stored over the tunnel This method of 
loading permits practically the entire length of train to be loaded 
at one time, but it is open to the disadvantage that a considerable 
amount of material is required in storage which can not be used 
for loading purposes. The material is simply piled over the tunnel 
and all of it that lies to the side of the tunnel chutes is practically 
dead, so far as loading is concerned, unless it be rehandled. Tun- 
nels are rather expensive to construct and this expense does not 
seem to be justified when the advantages of the tunnel method are 
compared with those of open bins holding two to three trainloads 
of material. Open bins with this capacity have successfully loaded 
trains where the maximum output with one mixer exceeded 1,200 
square yards of pavement, 8 inches thick, per 10-hour day, and 
where the average output was well over 900 square yards per day 
for weeks at a time. If two mixers are to be operated on a long- 
and-short haul basis from one central porportioning plant, rapid 
