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.

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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

CH. XXVIII.

CONDENSATION OF STEAM.

249

ice, E. Now as long as the tap, c, is kept open, the steam which is constantly rising from the water in B will rush along the tube into the empty flask, A, and will there be turned into drops of water by the cold of the ice underneath, and this will go on as long as there is any water left in B, because there will always be an empty space or vacuum in a to receive the steam as it rises. When the tap, c, is shut, the steam in в will become very dense, and when it is opened

A, Flask empty of air. B, Flask half-full of water and empty of air. c, Tap connecting the two bottles. D, Spirit lamp keeping the water in в boiling. E, Basin of ice cooling down the steam which passes into A.

again the greater part of the steam will rush out and be cooled down in A, while B remains hot as before.

Watt's Engine. This was exactly the plan Watt adopted in his steam-engine; B answers to his cylinder (Fig. 43), which could be kept always hot, and a to his condenser, in which his steam was turned back into water. We cannot follow out all the different steps of his invention, and must content ourselves with a rough description of his engine after he had completed it, as shown in Fig. 43.

In the first place you must notice that cold water is kept flowing down from the tank A into B, and out through the pipe C, so that the condenser standing inside в is kept quite cold:

and, secondly, I must tell you that the rods, I and 2, are so placed that when the engine-end of the lever-beam is raised, as in the figure, the stopcocks a and c are open, and b and d are shut; and when that end of the beam falls, b and d will be open, and a and c will be shut.

Let us now begin with the machine as we see it in the figure. In this position of the beam the cocks a and c are open; therefore, the steam below the piston will rush out at

A, B, Cold water tanks. C, Outlet for cold water. D, E, Pumps for drawing off hot water and sending it along s, s, back to the boiler. P, Tight-fitting piston. a, d, Cocks for letting steam into the cylinder. b, c, Cocks for letting steam_out of the cylinder. e, e, Pipe which carries steam from boiler to cylinder. o, o, Pipe which carries steam from cylinder to condenser. 1, 2, Rods connecting the cocks with the lever-beam.

c into the condenser, there to be turned into drops of water, while the steam from the boiler, entering at a, will force the

CH. XXVIII.

WATT'S ENGINE.

251

piston down. But now, the piston having pulled down the beam, a and c will be closed and the other two cocks, b and d, will be opened. So the steam above the piston will rush out at b into the condenser, while the steam from the boiler will pass directly from e down to d, and coming in below the piston, will drive it up again. In this way, although the cylinder is never cooled, the piston moves steadily up and down; because the steam is driven off into the condenser standing in B, where it is turned into water, and is drawn up by the two pumps D and E, and sent along the pipe, s,s, back to the boiler.

This was the principle of Watt's double-acting steamengine, and if you understand the difference between Figs. 41 and 43 you will see that, though Watt was not the first to make engines move by steam, he was the first to make a pure steam-engine, where the piston moves up and down without any help from the outside air, or of the counterbalancing weight e, Fig. 41, and without the enormous waste of heat and fuel which made all the earlier engines comparatively useless.

I have only told you here of the way in which he applied steam to his engines; all the numberless other improvements which he made you must read about in books on engineering. For twenty long years he went on improving and inventing without reaping any reward for his labour. Other men tried to steal his ideas and to make a profit out of his genius, and he had to fight against prejudice and injustice, and against constant depression caused by his own ill-health. Yet he found many kind friends upon his road, and amongst the most famous of these was Boulton, the Birmingham manufacturer, who became his partner in 1769, and stood by him manfully in all his difficulties and troubles. It was from

Boulton's manufactory at Soho (a suburb of Birmingham) that Watt's engines went forth to the world, and worked that great change in the manufactures of England which has made us one of the first nations of the world.

The names of Boulton and Watt deserve to be classed together as benefactors of mankind. Watt was the inventor, the man who loved science, and who could not live without creating. Boulton was the large-minded, enterprising man of business; he gave Watt men, money, courage, and sup port to carry out his inventions; and by his sympathy with, and command over the workmen, he led the army which conquered indifference, persecution, and difficulties, and established steam machinery in all the workshops of the world. Watt died in 1819, in the eighty-third year of his age, and was buried in Handsworth Church, near his friend and partner Boulton, who had died ten years before.

Chief Works consulted.—Black's Elements of Chemistry,' 1803; 'Edinburgh Review,' vol. xiii. 'History of Steam-engines;' Arago, Biographies of Scientific Men,' 1857; Smiles's 'Lives of Boulton and Watt;' Everett Deschanel's 'Natural Philosophy ;' Tyndall's 'Natural Philosophy;' Balfour Stewart's Treatise on Heat ;' Beckman's History of Inventions.'