7o 



POPULAR SCIENCE MONTHLY. 



dously the lapse of time necessary for the transference of raw material 

 from vessels to cars. 



In the case of all forms of bridge tramway apparatus it is necessary 

 to employ large gangs of men to fill by means of hand shovels the tubs 

 or buckets carried by the trolleys and naturally therefore there is in the 

 transportation world a tendency to regard with favor the latest inven- 

 tions in the line of machinery for the rapid unloading of iron ore, 

 namely, the automatic unloaders which dispense entirely with human 

 energy directly applied in the unloading operations. The fundamental 

 principle of all the automatic unloaders is found in the operation of 

 some sort of a clam-shell bucket which is let down into the hold of a 

 vessel with its iron jaws extended and, closing them, retains in its grasp 



Bridge Tramways for Handling Ore and Coal. Menominee Dock at Ashtabula. 



one or more tons of ore while it is lifted from the hold and run back 

 to a stock pile or waiting railroad cars after the manner of the bucket 

 of the bridge tramway. The original automatic unloader, introduced 

 only two or three years ago and in active use to-day, weighs several 

 hundred tons, and is equipped with a great mast to be lowered through 

 the vessel hatch and from which depends a clam-shell bucket capable 

 of holding ten tons of ore. The later patterns of unloaders, automatic 

 in their action, are fitted with excavating buckets of only about one ton 

 capacity, and which therefore permit of hoisting and transference by 

 wire cable instead of necessitating the ponderous iron and steel struc- 

 ture required to support the mast and clam-shell in the original design. 



