James Watt, 1736-1819.-Black had now completed his discovery, and from that time he taught in all his lectures that heat becomes latent or absorbed when a solid is changed into a liquid, or a liquid into vapour. It was about this time that the famous engineer, James Watt, began to study the power of steam, and as Black was his friend, he came to him to help him solve his difficulties. The history of the steam-engine, being the history of an invention, does not strictly belong to our work; but the use which Watt made of the discoveries about steam is a part of science, and we must therefore find room for a slight sketch of it here. James Watt was born at Greenock in 1736; he was the son of a builder and shipwright, and was so delicate as a child that he was kept at home, and learnt reading from his mother, and writing and arithmetic from his father. When at last he was sent to school he found. it hard work, for he was slow and thoughtful, and the other children jeered at him for his want of quickness. Everyone knows the story. of his being scolded by his aunt for sitting silent a whole hour, holding first a spoon and then a saucer over the steam rising from a kettle, and watching drops of water gathering upon them. It was in this quiet way that little James's mind grew, and it may be an encouragement to slow, plodding boys to know that one of our greatest inventors was considered a dull and backward child. As he grew older James went up to London, and there, after overcoming many obstacles, which the guilds, or trades' unions of those days, put in the way of all independent workers, he learnt to make mathematical instruments, and then returned to Glasgow, where he began business. Though he was only one-and-twenty he soon became known as a CH. XXVIII. JAMES WATT. 245 man of unusual ability, for the mind of the dull boy had developed, and his thoughtfulness had begun to produce results. Not only the students, but even the professors of the University used to stroll into his little shop to discuss the discoveries of the day. Whenever any difficulty arrested us,' writes a student named Robison, 'we used to run to our workman, and he never let go his hold until he had entirely cleared up the proposed question.' One day it was necessary to read a German book on mechanics; Watt immediately set to work and learnt German, and another time, for the same reason, he studied the Italian language. It is scarcely surprising that a man with such talent and perseverance as this, who was also gentle and loving to everybody, should be sought after both by masters and students. Among those who came to Watt's shop was one Anderson, professor of physics, who, finding that a little model of a steam-engine in the University museum was out of order, brought it to Watt to be repaired, and thus led the way to his invention. And here it is necessary to point out two things: First, you must not suppose that by a steam-engine is meant a railway engine; all contrivances which move by the power of steam are steam-engines, and locomotive engines which draw carriages were not made till 1804, long after Watt's time. Secondly, you must get rid of the idea, which many people have, that Watt was the first man to make an engine which moved by steam. This was done long before his time. The thing which Watt really did was to make an engine such as we now use, working entirely by steam, without the help of air, and doing an enormously greater amount of work with the same quantity of fuel than any others had done before. The Newcomen Engine, 1705.-Steam had been used to turn a globe by Hero of Alexandria, a Greek who lived 120 years before Christ; and Baptiste Porta in 1580, Solomon de Caus in 1615, and the Marquis of Worcester in 1663, all tried to make use of steam to do work. Again, in 1690 and 1698, a Frenchman named Papin and an Englishman, Captain Savery, tried to make steam-engines to raise water out of mines. But the only one of all these engines which we need describe here was that which fell into the hands of Watt, and which was made by a man named Newcomen in 1705. A plan of Newcomen's engine is given in Fig. 41. Its working depended on the pressure of the atmosphere (explained p. 123) on the piston at one end of the beam, and the weight of the lump of iron, e, at the other end. The lever-beam of this engine is balanced in such a way that when it is not at work the weight e pulls it down on the side away from the engine, and the piston, p, p, is drawn up to the top of the cylinder, as in the figure. To set the engine going a fire is lighted under the boiler, and the tap or stopcock, a, is opened, so that the steam rises into the cylinder, driving out the air through the air-vent, c. As soon as the cylinder is full of steam, a is turned off, and the stopcock, b, turned on. Immediately a small jet of cold water from the tank T rushes through into the cylinder, turning the steam back into a few drops of water, which flow out with the cold water down the pipe d. Now notice, the cylinder is quite empty; for the steam drove out the air, and the cold water carried the steam away with it, while no air can come in at c or d, because the little valves in them are kept shut by the weight of the atmosphere outside. So there is nothing to hold up the piston, which is being heavily pressed down by the air above it. The consequence is, down it comes to the bottom of the cylinder, dragging with it the CH. XXVIII. NEWCOMEN'S ENGINE. 247 end of the lever-beam. Directly it reaches the bottom the stopcock has to be shut, and a opened again. Up rises the steam directly from the boiler, driving up the piston, and the whole thing begins again. In this way the leverbeam is kept moving up and down by simply turning the a, Stopcock between boiler and cylinder. &, Stopcock between cold-water tank and cylinder. c, Valve closing air-vent. d, Valve closing the outlet for condensed steam. e, Weight which drags down the beam.,, Piston which is pressed down by the atmosphere when the cylinder is empty." two stopcocks one after the other. These were at first opened and shut by boys; but one day an ingenious lad named Humphrey Potter, who wanted to save himself the trouble of turning the cocks, found that by tying strings from The boiler and cold-water tank both in this figure and in fig. 43 are drawn much too small in proportion, in order to bring them into the figure. the handles to the different ends of the beam he could make the engine open its own cocks as the beam went up and down. This rough arrangement was soon improved, and the machine worked by itself. Watt's Separate Condenser.-Such was the engine as Watt found it. When he began to examine it he saw at once what an immense quantity of heat was wasted. Every time the piston came down, the cylinder, as well as the steam in it, had to be cooled down; every time the piston rose, the cylinder had to be heated again; and the thing which puzzled him most about it was, that it took six pounds of cold water to condense only one pound of steam. It was in this difficulty that he came to Dr. Black, and learnt from him the theory of latent heat, which showed that there is an immense store of heat hidden in steam, which has to be drawn out before it can become water. This was an entirely new light to Watt, and it led him to make many experiments still more exact than those of Dr. Black, which convinced him that no engine would ever work well or economically, while so much power was wasted in cooling and re-heating the cylinder at every stroke. But how was he to cool down the steam without cooling the cylinder which held it? answer. For months he pondered over this without finding any At last, one Sunday afternoon, when he was walking on the Green of Glasgow, the way to do it flashed upon his mind. If he could draw the steam off into a separate vessel and condense it there, the cylinder might still be kept hot, and the thing would be done. Fig. 42 will help you to understand how this could be effected. Here the two flasks, a and B, are first quite emptied of air, and B is half filled with. water. Under B is placed a lamp, D; under A, a basin of |