UTILITIES FOR HANDLING HAW MATERIAL. 69 



full height of the tramway or hy an automatic deflection of their 

 motion and with no appreciable loss of speed the buckets may be 

 caused to descend and discharge their contents at any point below the 

 tramway. 



The bridge tramways are usually built in plants or groups of 

 three or four bridges which may be supported on either single or double 

 piers. The tramways are, as a rule, provided with hinged aprons de- 

 signed to extend over the vessels and very frequently cantilever ex- 

 tensions are provided at the opposite end, so that the buckets are 

 enabled to serve a space of 300 to 350 feet in width. The operation of 

 the tub or bucket is effected by means of a wire cable connected with 

 a drum in the engine room, and the engine is usually of the double 

 cylinder type. The piers supporting a bridge tramway are on wheels, 

 and it is thus possible to skew or move sideways the entire structure, in 

 order to bring the bridges in line with the hatches of the vessel being 

 unloaded. This type of machine hoists the bucket of ore from the hold 

 of the vessel, conveys it to any desirable point on the tramway and 

 automatically dumps the material on the dock or into waiting railroad 

 cars. 



As indicating the capacity of the bridge tramway, it may be cited 

 that a plant of three bridges will readily handle 1,200 tons of ore in a 

 day of ten working hours, hoisting the material, conveying it a distance 

 of 100 to 150 feet and dumping automatically. In the case of the 

 bridges of exceptional length a 

 round trip can be made from the 

 hold of the vessel to the extreme 

 end of the cantilever and back 

 again, a distance of 600 feet in 

 one minute, and in actual opera- 

 tions a rate of 45 seconds per trip 

 has been maintained for hours at 

 a time. The buckets or tubs for 

 conveying ore are usually of one 

 ton or a ton and a half capacity. 

 There are several modifications of 

 the bridge tramway system, nota- 

 bly cable tramways in which wire 

 cables are substituted for the 

 bridges and what are technically 

 known as ' fast plants ' wherein in- 

 stead of the long bridges there are extremely short ones with tramway 

 projections over the vessel and cantilever extensions over the railroad 

 tracks on the dock, the effect of this short haul being to reduce tremen- 



Clam-shell Bucket of Automatic Unloader. 



