Floating Derricks. —An important class of transport machines are the floating derricks employed in rivers and harbors. These lunge appliances are capable of raising sunken vessels, of transferring heavy freight to or from clocks, of lifting and carrying blocks of granite in engineering works, and of handling heavy bodies whose transfer is not easily effected by other means. They usually consist of a large rectangular float or flat-bottomed boat well braced and stiffened by trusses; sometimes the float is divided into water-tight compartments which can be filled to counterbalance any weight on the opposite side. The float carries a pyramidal frame-work of strong timbers which supports an iron mast called a "king-post," and also the boom, a girder-like construction of sufficient length to give suit able horizontal clearance to the float. The hoisting machinery is placed under the tower, and is controlled by the engineer.
Elevators are of two general classes: (i) those for transferring pulveru lent or other loose materials, such as flour, grain, coal, bricks, mortar, etc., from one point to another vertically; and (2) those for raising or lowering goods to or from different floors or levels. To the latter class also belongs the passenger elevator which is provided for the convenience of persons ascending to or descending from the upper floors of hotels and office build ings. Elevators are operated either by hand, steam, hydraulic, or pneu matic power.
Elevators. —The grain-elevator, which was invented by Oliver Evans (see p. 43), is principally employed in flour-mills for the transfer of grain, flour, etc. from the lower to the upper part of the mill house. It consists of a strong belt carrying a series of metallic buckets or scoops and travelling over a revolving drum at each end. The buck eted belt is enclosed by a casing, usually of wood, in which the buckets pass freely, and as the buckets tip over on the upper drum their contents are discharged into a bin or a chute. The term " conveyer" is also given to such an appliance, but conveyers are more properly those machines which move materials horizontally. (See p. 31 x.) Ice-, brick-, and mor tar-elevators arc commonly constructed on the principle of the endless belt, but these are provided with endless chains or linked belts and slats suitably arranged for their different purposes.
Prokb1 Hoists or simplest type of a hoist for merchan dise consists of a winding drum or wooden cylinder for the rope or chain by which the article is lifted, and of a grooved pulley of large diameter over which an endless rope passes for operating the drum and effecting the lift. An improved form is the "double-lift" hoist shown in Figure 2 (pl. Ito), which has a chain, with a hook on each end, pass ing over a sheave turned by the hand-rope and wheel. Pulling the hand
rope on either side causes the opposite length of the chain to rise with its load, so that as one hook ascends the other hook descends ready for the next load. A safety-brake holds the load suspended if the hoisting rope be released.
Power effectiveness of hoisting apparatus was materially increased by the invention of the platform hoist, by which a greater number of articles could be raised at one time. This led to the application of a motive power other than hand-power, and there has resulted the modern warehouse and factory hoist (fig. 3), commonly ope rated by steam. The power elevator consists of a platform (II) or a cage suspended from one or more hempen or wire ropes passing over a sheave (C); the platform moves in guides or ways which are usually provided with ratchet-plates for engaging the safety-catches in case of breakage of the ropes, and thus preventing the precipitation of the platform. The hatchways or openings in the floors through which the goods are lifted or lowered are in some instances provided with automatically-operated doors, whose mechanism is of various forms, but which are in all cases worked by the elevator itself. Figure 4 exhibits an elevator steam-engine.
peculiar form of mechanical hoist, known as a "man engine," is employed in European mines for raising or lowering Nvorkmen. This apparatus was invented early in the present century by Dorrell of Clausthal (Upper Hartz), who made use of two reciprocating pump-rods, to which he fixed small platforms and handles on those parts of the rods that came opposite after each stroke, so that a workman by simply chang ing; his stand from one rod to the other after each stroke would be rapidly lifted to the surface. Figure 5 exhibits a man-engine on this princi ple, but instead of the pump-rods there are here provided two recipro cating timber rods, 8 inches square, driven by special machinery. The timber rods are furnished with stages or platforms at intervals of 12 feet, the distance travelled by each rod in its up and down stroke. A man stepping on the lower platform of the right-hand rod, for example, is raised 12 feet by the upward movement of the rod, which brings him to the level of the second platform of the left-hand rod, on which he steps, this rod being at its lowest position; the upward movement of the left hand rod lifts the workman again 12 feet higher, and so by stepping alter nately from one rod to the other after each of the reciprocating rods has made its stroke, he is hoisted out of the mine-shaft. The operation of lowering is the converse of the above. At one of the mines in Bohemia the man-engine reaches a depth of 2400 feet, and 300o men go up and down it daily in three shifts of eight hours each. In a modified form (pl.