are generally planted in groups of 3, 5, or 7, with battery connections leading back to a single junction-box containing as many disconnectors as there are separate mines. The purpose of this is, first, to centralize the connections of the mines, and then, by the separate disconnectors, to prevent the discharge of a single mine from short-circuiting and cutting out the rest of the group from subsequent control. The main multiple cable is led from the junction-box shoreward to the observation station, where each mine has suggest, and may be either spherical or cylindrical, as shown in the diagram. Being intended to explode only when in actual contact with the bottom of a vessel, they lie nearer the surface, and carry a moderate charge of 75 pounds of guncotton. No craft has yet been fashioned that can resist the under-water attack of such a weapon. These mines differ radically from the observation mine in their method of firing. Within a pedestal in their upper half, they carry the bared ends of the firing circuit-one at the top and one at the bottom. This pedestal is partly filled with mercury, which, when the mine is tipped about seventy degrees, bathes both of the bared ends of the firing circuit, thus forming a continuous metallic connection, and closing the circuit. The circuit so closed heats a platinum filament which fires a primer similar to that in the observation mine. GROUPS OF CONTACT MINES, PLAN VIEW. its button on an elaborate keyboard. These mines can be exploded either singly or in groups. The effectiveness of the observation mine depends upon accuracy in locating the enemy over them. This is done by cross-bearings, which, however, may be more or less indistinct or invisible by reason of atmospheric conditions. The act of discharge is effected by an electric current, which, heating a platinum filament in the primer, ignites the fulminate of mercury, which sets the explosive chain in motion. In principle of firing, the observation mine is not unlike the explosive sequence in the Whitehead torpedo. Until the firing key is pressed, the observation mine is pre-eminently safe and will stand a lot of rough handling, for the primer is pretty securely housed within the mass of the insensitive wet gun-cotton. Electro-Contact Mines The Electro-Contact mine is made either of iron or of steel, as conditions B. Cable End d Anchers Mines are distributed in groups and electrically controlled from shore. Vessels avoiding one group will certainly pass over another. Whole field is swept by battery fire, and illuminated by searchlights at night, to prevent counter-mining. The electro ready for instant service. chemical mine is similar in principle, differing only in the nature of the material of the circuit-closer. An acid is set free by the tilting, which, by coming in contact with the two elements of a battery attached to the ends of the firingcircuit wires, creates a current strong enough to heat the platinum filament of the primer. Naval Defense Mines There is a form of electro-contact mine now carried by the heavy fighting ships of a number of nations, called the "Naval Defense" mine. These mines are intended to protect a hastily established naval base on the enemy's coast or at some point not otherwise defended. They differ from the ordinary electro-contact mines only in the absence of a shore-battery connection. After these mines are planted and the mine-laying vessel or boat is at a safe distance, a small battery, in a water-tight case and connected by a number of yards of small, insulated cable, is thrown overboard; and thenand then only-the mine becomes dangerous to any craft hitting it. To recover these mines in safety, the battery-boxes are swept for, raised, and disconnected, after which the mines themselves may be recovered without danger. Mines of this sort can be laid and recovered very rapidly, even during the heat of action; and, by an ingenious arrangement, they are made self-anchoring at a predetermined depth. Counter-Mining Counter-mining is the work of rendering an enemy's mine fields ineffective, either by cutting his electric cable connections or by exploding the mines by the shock of counter-mines fired in their neighborhood. This work involves a great deal of risk, is very hazardous, and must be carried out under cover of night or fog and with the utmost deliberation. The chances of its success are reduced to a minimum in the face of the powerful electric lights and the rapid-fire guns that should always command the approaches to the mine fields. Men there are in plenty who can be found at all times to undertake the tasks of such forlorn hopes, and in that readiness we see how little the awfulness of modern weapons of war will halt a gallant and persistent foe. Automatic Appliances for Handling Coal and Iron Ore A Description of Some of the Most Important Modern Adjuncts of the Lake and Railway Carrying Trade I' By WALDON FAWCETT T IS SAFE TO SAY that in no other one division of the commercial and industrial world has the past quarter of a century witnessed the rapid, continuous, and truly phenomenal development which has characterized the evolution of means and methods for handling coal and iron ore in the transit of these most important of raw materials from mine to furnace, or, in other words, from Mother Earth to the finished product. To so high a state of perfection have the utilities employed in the handling of these bulk commodities been brought, that the dawning years of the twentieth century find practically the entire movement from mine to market accomplished solely by mechanical means. Surpassing as the achievements of the past score and a half of years have been in labor-saving, and consequently in the cost of handling iron ore and coal, they have not been more wonderful than the economies of time effected by the more rapid handling of the world's two most important minerals. It is indeed a far cry from the old method of trundling small loads of the bulk material back and forth in wheelbarrows, to the new plan of transferring a carload of coal per minute or to the moving of ore by means of grab buckets that scoop up automatically anywhere from two to ten tons at each operation; and yet, as has been said, less than thirty years has been required to work this amazing transformation. The conditions governing the movement of iron ore and coal in the United States are peculiar in that several rehandlings are necessary. The major portion of the iron ore is mined in the States of Minnesota, Wisconsin, and Michigan; transported by rail to ports on Lakes Michigan and Superior; and then loaded on vessels which in some instances carry it to blast furnaces located directly on the shores of the inland seas, iron ore, and are subject to the same rehandlings. The pioneer inventor who successfully solved the problem of handling coal and iron ore over a considerable range of distances solely or largely by mechanical means, was Mr. Alexander E. Brown, the well-known American engineer, the machines of whose invention are to this day accepted as the standard-and are indeed the sole-means of performing many of the functions in this highly specialized field. Bridge Tramways At the outset all the operations of transferring iron ore from ships to cars BRIDGE TRAMWAYS FOR HANDLING ORE AND COAL. but which more frequently transport it to harbors on the south shore of Lake Erie, where there is a transfer from the vessels to railroad cars, in which the ore is moved to the blast furnaces of that great ironmanufacturing section known as the "Pittsburg District." Here, finally, there is still another rehandling when the ore which has been placed in stock piles is, as occasion requires, conveyed to the furnaces. The anthracite coal from the Pennsylvania field, and the bituminous fuel from the mines of Ohio, Pennsylvania, and neighboring States, which are shipped to points in the West and Northwest, traverse in the opposite direction practically the same route followed by the or stock piles, as the same might be, and of both loading and unloading coalcarrying vessels to and from cars or stock piles, were confined to a single class of apparatus-hoisting and conveying machinery of what is known as the "bridge tramway" type; and this form of handling appliance is yet so extensively used in moving both commodities as to merit first attention aside from its claim by reason of priority of invention. A bridge tramway is, in effect, a miniature elevated railway along which loads are conveyed by the trolley system now so extensively employed in the industrial field. The skeleton bridge structure-supported at both ends by piers-is built of iron and steel designed to give the maximum strength with the minimum weight of material, and with all members made of such shapes, and so arranged in the trusses, as to offer the least possible surface to wind-pressure an important consideration owing to the exposed locations in which these machines are generally used. The bridge tramways proper usually range from 180 to 192 feet span; but, extending from the front pier, is a 34-foot apron reaching from the front of the dock out over the vessel tied up for unloading, while from the rear pier is a cantilever extension stretching back from 80 to 104 feet additional. Thus an area more than 300 feet wide is served by the tramway. The piers are of steel construction, and are high enough to support the bridge on an incline with the front end about 27 feet above the ground and the rear end 52 feet above the level. The piers are mounted on wheels running on tracks, so that the whole structure may be "skewed" or moved sideways back and forth along a dock to suit the hatches of a vessel. Thus a vessel may be unloaded by having several bridge tramways operating simultaneously over as many different hatchways or openings in the deck; or the cargo may be removed by a single tramway adjusted so as to operate in first one division of the hold and then another, by means of the sidewise movement of the apparatus. Running along a track suspended from the bridge between the girders, is a trolley with suspended bottom block and hook to which is attached the hoisting or pulling line, and all the motions of which are under perfect control of the operator by means of suitable levers. Up and down this trolley line, at a speed of hundreds of feet per minute, travels an iron tub or bucket in which the ore or coal is carried. These buckets are made in various sizes; but what might be termed the standard size has a capacity of seventeen cubic feet (or a gross ton) of ore. The plan of operation is practically the same in all cases. If coal or iron ore is being unloaded from a vessel, the operator, upon receiving from the hold of the vessel a signal that a tub is filled, throws "CLAM-SHELL" BUCKET FOR AUTOMATIC UNLOADING. Showing Operator in Mast. of the bucket, allowing it to upset, discharge its load, and automatically right itself. As soon as this is done, the operator releases the lever, and the bucket returns by gravity to its starting point in the hold of the vessel. The block unhooks automatically from the trolley, allowing the bucket to be lowered into the hold, whereupon the empty bucket is unhooked and a filled one hooked on. If preferred, the operator, instead of allowing the bucket to dump automatically, can lower it to any desired point for discharge, this being desirable when it is the purpose to transfer the coal or ore directly to waiting railroad cars. One of the buckets such as have been described, will make a round trip from the hold of a vessel to the end of a bridge tramway trolley line, and return-a distance of 600 feet-in one minute; and in actual work a rate of forty-five seconds |