NEWCOMEN’S ENGINE
To work it, steam was generated in the boiler at a pressure slightly greater than atmospheric. By the opening of a cock steam was admitted to the cylinder, below the piston, which was initially at rest in its highest position. The steam having filled the cylinder and expelled nearly all the air, the cock was shut and the cylinder was chilled by an external spray of cold water. Whereupon, as soon as the steam in the cylinder began to condense, the piston, forced down by the now unbalanced atmospheric pressure above it, began to descend. As soon as it had completed its downward stroke steam was again admitted beneath the piston, and, the pressure on the two sides of the piston becoming equal, the piston began to move up again to its original position. And so on.
This was the original Newcomen engine. Even in this primitive form it far surpassed Savery’s in economy of fuel and in safety. It had, too, far greater flexibility in the manner in which its power could be applied; it could be used not only to lift a certain volume of water through a relatively small height, but a smaller volume through a greater height: which was a desideratum in the case of deep mines like those of Cornwall. In 1720 an engine was erected at Wheal Fortune mine having a cylinder nearly four feet in diameter and drawing water, at fifteen strokes a minute, from a depth of 180 feet.
Yet it was apparent that the engine was in many respects inefficient. The cocks, for instance, which controlled the motion of the piston had to be opened and shut by a man. Sometimes he let the piston rise too far, in fact, right out of the cylinder; sometimes he let it down too fast, so as to damage the engine. Again, the external spraying of the cylinder at every stroke to induce condensation of the steam within was an obviously clumsy and primitive operation. It was not long before external spraying gave place to internal cooling of the steam by the injection of water; this method being discovered, it is said, as the result of a leaky piston allowing its sealing water to pass, yet giving unaccountably good results. The difficulties with the cocks were overcome by the laziness or initiative of a youth named Humphrey Potter, who attached some strings and catches to the cocks of an engine which he was employed to work at Wolverhampton.[77]
With these improvements the engine remained practically without alteration for the next forty years. Its greatest sphere of usefulness was in the northern coalfields, where cheap and abundant fuel was close at hand. In Cornwall, until by special legislation the duty on seaborne coal was remitted when used for Newcomen’s engine, the cost of fuel proved a great obstacle to its use.
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In 1764 James Watt, an instrument maker employed on work for Glasgow College, was given the task of repairing a working model of a Newcomen engine.
A man of serious and philosophical mind, an intimate friend of Professor Robison, the physicist, and acquainted with the famous Dr. Black of Edinburgh, then in the thick of his researches on the phenomena of latent heat, Watt often discussed with these two scientists the possibility of improving the steam engine; which apparatus was still only employed for the purpose of pumping water, and which was so clumsy and so wasteful of fuel as to be comparatively little used. To this end he was induced to try some experiments on the production and condensation of steam. The results of these, and a knowledge of the newly discovered phenomenon of latent heat,[78] convinced him that the existing cycle of operations in the engine was fundamentally inefficient, and that improvement was to be sought in the engine itself rather than in the boiler, which was the element which was receiving most attention from contemporary investigators.
In particular, he clearly discerned the thermal inefficiency of the Newcomen engine: the waste of heat involved in alternately heating and cooling the large metal cylinder, which absorbed such immense quantities of fuel. Watt’s first idea was, to lag the cylinder in wood so as to prevent all outward radiation. But the result of a trial of a lagged cylinder was disappointing. A gain was certainly obtained in that the steam, when admitted to the cylinder, did not require to raise by partial condensation the temperature of the walls; it exerted its expansive force at once and the piston rose. But on the other hand much greater difficulty was experienced in condensing it when a vacuum was required, for the down stroke. Moreover it was observed that an increase in the amount of injection water only made matters worse.