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at the feed-pipes. It would not give way under any circumstance, not even though the valves were choked, the pressure is so extremely small; the boilers are calculated to sustain fifty times the pressure required of them. If any part of the boiler, by length of use, became very thin, and gave way, it would merely rend, if malleable iron. The accidents that happened from boilers, sometime ago, were owing to their being on the high-pressure principle, and being made of cast iron. When made of cast iron, and the pressure too high, they gave way, and the metal flew just like a shell. Malleable iron tears and rends, without flying or doing injury. The experiments made in the river with the Meteor, shewed that she consumed about seven bushels per hour, rather under, she was then working above her full speed: the Sovereign was from nine and a quarter to nine and a half; the Meteor has two thirty-horse engines, and the Sovereign two forty-horse engines; the latter when using that quantity of coal, was going about nine miles and three quarters through the water per hour. A bushel of good Newcastle coals is reckoned equal to a hundred weight of Scotch coal, so that it comes to very nearly the same thing; the Scotch coal generally burns very free, and so does the Staffordshire; but the bushel of Newcastle coal is equal to a hundred weight of either.The standard bushel should weigh eighty-eight pounds; the best Walls End eighty; and the Wylam seventyseven; found that by actual weight, the specific gravity of the Wylam coal was much under that of the Walls End, the latter was not good for working engines. The best coal for working engines is the Halbeath or Inverkeithing, from a place in Fife called Inverkeithing.Their peculiar quality is that of burning free, and becoming completely white ashes, without caking upon the

fire bars.—It is a great object to get good coals. In the first passage that I went with the City of Edinburgh from London to Leith, the coals were very indifferent, and we were obliged to clear the fire every four hours. In coming back, we had a description of coal called Halbeath Main, and with them went sixteen hours without cleaning the grates; the sulphur in coals will destroy the fire bars in a short time.

CHAPTER V.

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Cylinder and Piston-Condenser -Air-Pump-Barometer and Steam-Gauge- Working Beam-Parallel Motion Counter Sun and Planet Wheel - Fly and other Modes of regulating Velocity-Governor— -Boiler-Safety-Valves-Furnace.

HAVING taken a brief review of the early history and general principle of this stupendous machine, it may be advisable before we proceed to a description of the principal engines now employed, to examine more minutely the separate parts and the progressive improvements effected in each.

The cylinder and piston being those parts of the engine in which the effective force is more immediately produced, may first claim attention.

The piston of the atmospheric engine is generally made of cast iron nearly fitting the inside of the cylinder, a circular ledge or rim being formed round it to receive the packing, without which the steam would find a passage through the interstices in the cylinder. Mr. Smeaton, who greatly improved the atmospheric engine, coated the under side of the piston with elm or beech planks about two inches and a quarter thick; the wooden bottom being screwed to the iron with a double thickness of flannel and tar, to exclude the air between the iron and the wood. By the adoption of this improvement its property of conducting

heat was diminished, and the wood having been previously jointed with the grain radiating in all directions from the centre, was not liable to expand very materially by the heated steam. This piston was kept air-tight by a small stream of water continually falling on its upper surface; but in Mr. Watt's engine he was compelled to effect this by improving the fitting of the piston, the old mode being inadmissible.

It is now cast with a projecting rim at bottom, which is fitted as accurately to the cylinder as it can be, to leave it at full liberty to rise and fall through the whole length. The part of the piston above the rim is a little less all round than the cylinder, to leave a circular groove for the hemp which forms the packing. To keep this in its place, a lid or cover is put over the top of the piston, with a ring or projecting part, which enters into the circular groove for the packing, h h, and pressing upon it the plate is forced down by screws, ii, which work into the body of the piston at a. By this means the packing is made to fill the diameter of the cylinder with tolerable accuracy, and to prevent for a time any steam passing between the piston and the cylinder. The connection with the piston-rod is seen at rc. When, however, by continued working the piston became too easy, and so occasioned a waste of steam, it was found necessary to take off the top of the cylinder to get at the screws, even when fresh hemp or packing was not wanted, and this operation being attended

with considerable labour, was seldom resorted to by the engine-man till a great waste of steam had taken place. By an improvement on this piston introduced by Mr. Woolf, this is now effected without taking off the cylinder cover, except, indeed, when new packing is required.

To accomplish this, Mr. Woolf fastens on the head of each of the screws a small cog-wheel or nut, and these are all connected together by means of a central wheel working loose upon the piston-rod in such a manner, that if any one of the screws be turned a similar motion is given to the remainder, a small cap being provided in the upper end of the cylinder screwed down by bolts to make it steam tight. In a piston thus constructed, there is little difficulty in drawing down the packing, by applying a key to the square head of the projecting screw employed to communicate with the rest. Another method contrived by Mr. Woolf for the smaller pistons differs but little from the preceding in construction. Instead of having several screws all worked down by one motion, there is in this but one screw, and that one cut upon the piston-rod itself; on this is placed a wheel, the centre of which is furnished with a female screw, which is forced down by means of a pinion furnished with a square projecting head turned in a similar manner to the preceding.

For high-pressure engines, however, the metallic piston invented by Mr. Cartwright has the most decided preference. This not only saves the trouble and expense of packing, which must be frequently renewed in all other engines, but also a great deal of steam, on account of the more accurate manner in which it is made to fit the cylinder; this is effected in the following manner: Two metal rings are accurately ground into the cylinder, so that no steam can pass between their exterior surface and

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