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Yet he did not make it. He was still unable to dispense with the condensing and vacuum and air-pressure ideas. Acting for the first time in the line of real efficiency, he failed to go far enough to attain it. He made a double-acting engine by the addition of many new parts; he even attained the point of applying his idea to the production of circular motion.
In 1787, Oliver Evans introduced improvements in grain mills, and was generally efficient as one of the beginners in the field of American invention. Soon afterwards he is known to have made a steam-engine which was the first high-pressure double-acting engine ever made.
Their only purpose was to get the piston lifted, and air-pressure did all the actual work. If you turn the tube, and put an air-pressure first at one end and then at the other, and pay no attention to vacuum or atmospheric pressure, you will have the principle of the later modern, almost universal, high-pressure, double-acting steam-engine.
By subjecting animals to death from the vapor of chloroform in the same atmosphere, and treating one set by artificial respiration with the double-acting pump, and the other set by artificial respiration excited by galvanism, I found that the first would recover in the proportion of five out of six, the second in proportion of one out of six.
The initial air cylinder is made large enough to correspond with the capacity of the smaller double-acting cylinder. The strains are equalized because the area of the large cylinder multiplied by its low pressure is exactly equal to that of the small cylinder multiplied by its high pressure.
The two bellows are then laid valve facing valve, and are attached to one another by a strip of tin passed round the wood just behind the nozzles and by tying the two fixed handles together. Double-acting bellows.
This differs from the one just described chiefly in that it is single-acting, while the other is double-acting. By single-acting is meant that the air cylinders compress their respective volumes of air once every revolution.
Suppose the tank should, through an accident, be empty, and you wanted to start?" "We've provided for that" "I expected so. Wabbling wheels of Wisconsin, you fellows are certainly wonders." "Nothing very wonderful about it," disclaimed Tom. "Well, if we find the tank is empty we have a powerful, double-acting hand pump by which, without much effort, we can get up any pressure we need."
He also employed a gas and air pump, which were placed respectively on either side of the engine cylinder, communication being established between the receiver into which the pumps delivered and the working cylinder as the charge was fired. The double-acting engines which Barnett devised later were not so successful. From this time to about 1860 very few practical developments are recorded.
It has 41 stops, 2,133 pipes, four sets of manuals, each manual with a compass of 61 notes; there are 30 pedal notes, 9 double-acting combination pedals; all the metal pipes are 75 per cent pure tin. In loving Christian fellowship the chorus abides. No difficulty that could not be settled among themselves has ever rent it; no jealousies mar its peaceful course. Professor Wood is a wise leader.
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