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.