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an evaporation take place, and the water in the boiler sink below its proper level, the stone also must sink, which will cause the valve to open wider, and let that from the cistern come in faster. If, on the other hand, the evaporation be less than it ought to be, the water will have a tendency to rise in the boiler, and with that the stone must rise, and the valve will, consequently, let the water in with less velocity. By this neat contrivance, the water in the boiler is always kept at one level.

Emma. What are the pipes t and u for?

Father. They are seldom used, but are intended to show the exact height of the water in the boiler. The one at t reaches very nearly to

the surface of the water when it is at the proper height: that at u enters a little below the surface. If then the water be at its proper height, and the cocks t and n be opened, steam will issue from the former, and water from the latter. But if the water be too high, it will rush out at t instead of steam: if two low, the steam will issue out of u instead of water.

Charles. Suppose things to be as represented in the plate, why will the water rush out of the cock u if it be opened? it will not rise above its level.

Father. True but you forget that there is a constant pressure of the steam on the surface of the water in the boiler, which tends to raise the water in the pipe u. This pressure would force the water through the

pipe, as in an artificial fountain. See p. 73 and 74.

Emma. You said Captain Savery was the inventor of the steam-engine. Father. His invention went merely to raising water from pits and mines. But, in its present improved state, the steam-engine is applied to a thousand useful and important purposes.

NOTE. In the next Conversation will be given an account of the purposes to which the steam-engine is applied. But perhaps one of the most striking exhibitions of the wonderful effects of this machine is to be seen in that part of the Portsmouth dockyard in which the blocks for ships are made. These blocks are completely finished from the rough timber, with scarcely any manual labour, by means of different saws and other tools worked by the steam-engine.

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CONVERSATION XVIII.

Of the Steam-Engine, and Papin's Digester.

CHARLES. We have seen the structure of the steam-engine and its mode of operation; but you have not told us the uses to which it is applied.

Father. The application of this power was at first wholly devoted to the raising of water, either from the mines, which could not be worked without such aid, or to the throwing it to some immense reservoir, for the purpose of supplying, with this useful

article, places which are higher than the natural level of the stream.

Emma. Is it to this that Dr. Darwin alludes in the lines,

Here high in air the rising stream he pours, To clay-built cisterns, or to lead-lin'd tow'rs; Fresh through a thousand pipes thewaves distils, And thirsty cities drink th' exuberant rills.

Father. It is; and you might have repeated the whole passage, in which the steam-engine, represented as a giant-power, is supposed applicable to the bringing up of the coals, and other ore from the mine, and to the working of the bellows at the furnace, in which the ore is melted:

Fan the white flame, and fuse the sparkling

ore.

The author refers also to the application of this engine to various other

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