Turbines Reversed.
For many years turbines have proved themselves better than other water-wheels, so that wherever an old-fashioned breast-wheel still goes its creaking round, there the sketcher seizes the picturesque outlines of a motor whose remaining days are few. A turbine in carefully curved vanes gets from falling water all the power it holds; when the task is to lift water, then this very turbine, reversed in direction, is the Worthington pump, the most efficient water-lifter known. The rules for construction are the same whether we start with falling water and derive power from it, or begin with power and raise water thereby. Quite as pictorial as a breast-wheel is a wind-mill, the older the better, thinks the artist as he views its weather-beaten frame. Much later than the wind-mill as a device is its counterpart, the fan-blower; the lines most effective for the one are also best for the other. Much more effective than the old-time mills of but four arms are new mills whose whole circle is covered by blades. Fan-blowers with a like multiplicity of vanes, yield most duty.
Hydraulic Pressure as a Counterbalance.
For ages one of the observations of every day has been that a column of water exerts pressure in proportion to its height. Usually this pressure is thought of as being exerted downward, but if a pipe, filled with water at great pressure, be curved upward at its base, then the contained liquid presses upward. Mark the gain of thus varying a little from the ordinary view point of a case. In 1883 Mr. J. F. Holloway, of California, set up a turbine with its stream admitted from below and moving upward through the vanes of the machine. He thus obliged the water pressure to aid in supporting the wheel, materially diminishing its friction through thus counterbalancing its weight. This plan has been adopted at Niagara Falls for the gigantic turbines there erected, among the most powerful in the world.
Engine and Pump.
That simple appliance, a garden squirt, exemplifies two important kinds of apparatus, one the converse of the other. Fill the cylinder with water, force the piston along its course, and you have a pump. Admit water under pressure, as from a city faucet, and it drives the piston of a motor; in principle such is the mechanism of thousands of motors in London, using water under a pressure of 500 pounds, or so, to the square inch. An apparatus, essentially the same, when supplied with steam or gas becomes the familiar engine at work in uncounted factories and mills. It was a great advance in steam engine design when the single cylinder of Watt was replaced by two or more cylinders, using steam at high instead of low pressure. Thus apportioned in a series of cylinders the steam is not nearly as much cooled, with loss of working power, as when but one cylinder is used. So likewise, it is best to divide the compressing of air into two or more stages, so that at each stage the air may be cooled, and thus more easily compressed than if a single operation completed the business. The best air compressor is virtually the converse of a steam engine.
Of late years reciprocating machinery, of one kind and another, has had to give place to rotary designs. In these, as in their predecessors, are striking cases of rules that work both ways. If steam at high pressure is fully to yield its energy in a Parsons-Westinghouse turbine, for example, the vanes must be rightly curved, and there must be a succession of them in circles gradually widened so that the steam may part with its energy, a step at a time. In mining, in metallurgy, in many another great industry, compressed air is required in huge volumes. For its production Mr. Parsons has invented an apparatus virtually the twin of his steam turbine, only that it runs in a reversed direction; it may be directly yoked to a steam turbine.
Fans.
Currents of air much less forceful than those of steam in a turbine are generated by the electric fans of our shops and offices. When their vanes move as the hands of a clock, a breeze comes toward you; reverse their motion and the stream blows away from you. Place such a fan in the side of a box otherwise closed; driven in one direction the vanes force air into the box; driven the opposite way the vanes remove air from the box. Powerful currents of this kind, such as stream from a Sturtevant blower, are used for blast furnaces and the largest steam installations. The engineer chooses between two methods; he can seal up the fire-room and force in air which will find its way through the grate-bars to the fuel, or he places a fan in the smoke-stack to induce a current by exhaustion. In New York and London underground pneumatic tubes carry letters to and from the post-offices. When the central engine works its fans exhaustively, water may be drawn into the tubes from the streets so as to do much harm. When the ground is thoroughly dry it is best to exhaust the air at one end of the line and compress it at the other. This union of a push and a pull resembles Lord Kelvin’s plan in ocean telegraphy, by which a cable is first connected with the negative pole of a battery and then, for a signal, made to touch the positive pole. With its path thus cleared, a message pulses along at a redoubled pace.