fig. 6) a single rod is used, one set of steps being on the vertically-recip rocating- rod, and two sets of landings being fixed on the side of the shaft, one set on each side of the rod. The workmen ascending occupy the set of landings on one side, while those descending occupy the opposite side, thus obviating any confusion in stepping on or off. Thus each step of the rod carries a man iu rising, and in descending likewise carries a man, one set of men landing on the platforms on one side, and the other set landing on the opposite side. A string of men thus ascend and descend merely with the labor of stepping on and off as the rod rises and sinks.
Passenger sites in the business parts of large cities now command almost fabulous prices, in consequence of which their owners are generally erecting taller buildings than formerly, as may be seen in New York, Philadelphia, Chicago, and other business centres of America and of Europe. The available space for offices, etc. is thus made to double its capacity, by the upward extension of the building, in a manner that would have been regarded as useless thirty years ago. The construction of many-storied edifices has been made feasible by the introduction of improved elevators, by which access is afforded to the several floors of a building without the loss of time or the severe exertion required to mount long flights of stairs. The complete system of machinery represented by the modern passenger elevator has, however, been brought to the present state of absolute safety, ease of operation, and thorough efficiency only through a long course of close observation and careful experiment.
Hydraulic elevators may be operated by steam, gas, or electric engines, or by hydraulic or pneumatic pressure. While the steam elevator is preferred for heavy and continuous duty or for freight purposes, the hydraulic elevator has practically supplanted all others for passenger service. The principle on which it is operated will be understood by the following description and by reference to Figure I (pi. III): The carriage is suspended by four or six wire ropes which pass over a sheave fixed above the highest point of the lift, thence under a pulley connected with the piston-rod of the cylinder whose weighted piston nearly counter balances the weight of the car. One of the ends of the series of wire ropes is fastened at the same level with the overhead sheave, and the other end to the bottom of the car-frame. The cylinder is composed of several cast-iron
sections bolted together through their flanged ends, and is vertically set at the lowest point of the elevator well. Leading to the upper part of the cylinder is a supply-pipe, and connected with the bottom of the cylinder is an eduction-pipe, both having suitable gates and valves for the supply and discharge of the water.
The operation of the elevator is as follows: The car being at the bottom of the shaft, the piston is in position at the upper end of the cylinder. The water under pressure, either from the street-main or from a tank on the roof, is admitted above the piston, which is forced down in the cylinder and causes the car to rise, the water in the cylinder under the piston being permitted to flow out at the bottom at the exact rate at which it enters at the top. In this way the atmospheric pressure, as well as the weight of the column of water above the piston, is utilized to maintain a con stant pressure upon the descending piston, which pressure is equal to the hydraulic head from the surface of the water in the upper res ervoir to the lower or discharge orifice of the cylinder. The cyl inder, however, is always full of water, the escapc-valve at the bottom being open only when the piston is falling and the water is coming in at the top; when the car is going down and the piston rising, the bottom escape-valve is closed, the water being simply forced by the rising piston through a circulating-pipe from the top of the cylinder into an opening at the bottom, thus being only transferred from above to below the piston; the car and piston cannot move faster than the gates and valves will allow this flow to take place. The valves can be so fixed that the speed of the car can be exactly regulated independent of the will of the operator in the car. As a further provision against too rapid movement, the passage of the elevator actuates a governor which can be set to regulate the speed as may be desired. If the starting-rope breaks or becomes detached, so that the valves cannot be closed by the operator, the piston can only descend to the lower part of the cylinder, raising the car to the top of the lift, where it will be held safely until the rope can be readjusted or the valves be opened by hand. The most remarkable application of the hydraulic lift is exhibited by the elevators of the Eiffel tower.