Page:Popular Science Monthly Volume 62.djvu/76

70 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 inventions 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

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