Page:Popular Science Monthly Volume 62.djvu/73

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inland seas wherein the iron ore is conveyed from the Lake Superior mining district to the ports adjacent to the blast furnaces of the Middle West are employed the various forms of hoisting and conveying apparatus, all of American origin, which probably constitute the most famous of all the installations for transportation purposes. In this field of activity methods advanced, at a single step, from the old plan of unloading the vessels by means of wheelbarrows and permanent trestles to the bridge tramway structures which are up to the present date in almost universal use.

The conspicuous elements in any such installation embrace the elevated tramway—spanning the dumping ground or railroad yard and

PSM V62 D073 Steam shovel.png

Steam Shovel.

connecting it with the vessels—the trolley or carriage traversing this tramway and the system of mechanism by which the whole is operated and controlled. Such an apparatus is operated, of course, by a motive power located beyond the limit of travel, and while the operation is at every stage subject to the control of an operator, a large proportion of the important functions are automatic, the positive movements of the parts through such operations being derived entirely from the bodily movement of the apparatus itself, while actuated by momentum, gravity or the direct action of the hoist rope. Attached to the trolley of each machine is an automatic dumping-tub or bucket, the discharge of which may be made at the will of the operator, either at the