POETLAISTD 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 to 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 T^his 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 



