Читать книгу Historic Inventions - Rupert Sargent Holland - Страница 7

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It was no pressing need that drove John Gutenberg to the invention of his printing press, nor was it necessity that led to Galileo’s discovery of the telescope, but it was a very urgent demand that led to the building of a steam-engine by James Watt. England and Scotland found that men and women, even with the aid of horses, could not work the coal mines as they must be worked if the countries were to be kept supplied with fuel. The small mines were used up, the larger ones must be deepened, and in that event it would be too long and arduous a task for men and women to raise the coal in small baskets, or for horses to draw it out by the windlass. A machine must be constructed that would do the work more quickly, more easily, and more cheaply.

A Frenchman named Denys Papin had built the first steam-engine with a piston. He had seen certain experiments that showed him how much strength there was in compressed air. He had noticed that air pressure could lift several men off their feet. His problem therefore was how best to compress the air, or, as it appeared to him, how to secure a vacuum. His experiments proved that he could do this by the use of steam. He took a simple cylinder and fitted a piston into it. Water was put in the cylinder under the piston, a fire was lighted beneath it, and as the water came to the boiling point the piston was forced upward by the steam. Then the fire was taken away, and as the steam in the cylinder condensed, the piston was forced down by the air pressure above. He fastened the upper end of the piston to a rope, which passed over two pulleys. If a weight were hung to the other end of the rope it would be raised as the piston was forced down. In that way the air pressure did the work of lifting the weight, and the necessary vacuum was obtained by forming steam and then condensing it in the cylinder. This was a very primitive device, requiring several minutes for the engine to make one stroke, but it was the beginning of the practical use of steam as a motive power.

Thomas Newcomen, an English blacksmith by trade, first put Papin’s idea to use. Instead of the rope and pulleys Newcomen fastened a walking-beam to the end of the piston, and attached a pump-rod to the other end of the walking-beam. He used the steam in the cylinder only to balance the pressure of the air on the piston, and let the pump-rod descend by its own weight. As the steam condensed the piston fell, and the pump-rod rose again. By this means he could pump water from a mine, or lift coal. His first engine was able to lift fifty gallons of water fifty yards at each stroke, and could make twelve strokes a minute. At first he condensed his steam by throwing cold water on the outside of the cylinder, but one day he discovered that the engine suddenly increased its speed, and he found that a hole had been worn in the cylinder, and that the water with which he had covered the top of the piston was entering through this hole. This condensed the steam more rapidly, and he adopted it as an improvement in his next engine. A little later a boy named Humphrey Potter, who had charge of turning the cocks that let the water and steam into the cylinder, found a way of tying strings to the cocks so that the engine would turn them itself, and so originated what came to be known as valve-gear.

Newcomen’s engine was a great help to the coal mines of England and Scotland, but it was very expensive to run, a large engine consuming no less than twenty-eight pounds of coal per hour per horse-power. Then it happened that in 1764 a small Newcomen engine that belonged to the University of Glasgow was given to James Watt, an instrument-maker at the university, to be repaired. To do this properly he made a study of all that had been discovered in regard to engines, and then set about to construct one for himself.

There are many stories told of the boyhood of James Watt. He lived at Greenock on the River Clyde in Scotland, and was of a quiet, almost shy disposition, and delicate in health. He was fond of drawing and of studying mechanical problems, but rarely had much to say about his studies. The story goes that as he sat one evening at the tea-table with his aunt, Mrs. Muirhead, she said reprovingly to him, “James Watt, I never saw such an idle boy: take a book or employ yourself usefully; for the last hour you haven’t spoken a word, but taken off the lid of that kettle and put it on again, holding a cup or a silver spoon over the steam, watching it rise from the spout, and catching the drops it falls into. Aren’t you ashamed of spending your time in this way?” And history goes on to presume that as the boy watched the bubbling kettle he was studying the laws of steam and making ready to put them to good use some day.