ful engineer, or other person conversant with the subject, who should ascertain, by trial, the strength of such boiler, and should certify his opinion of its sufficient strength, and of the security with which it might be employed to the extent proposed:-That every such boiler should be provided with two sufficient safety-valves, one of which should be inaccessible to the engineman, and the other accessible both to him and to the persons on board the packet:-That the inspector shall examine such safety-valves, and shall certify what is the pressure at which such safety-valves shall open, which pressure shall not exceed one-third of that by which the boiler has been proved, nor one-sixth of that which by calculation it shall be reckoned able to sustain :That a penalty shall be inflicted on any person placing additional weight on either of the safetyvalves.

4. Resolved, That the Chairman be directed to move the House, that leave be given to bring in a Bill for enforcing such regulations as may be necessary for the better management of steampackets, and for the security of his Majesty's subjects who may be passengers therein.

CHAPTER V.

Cylinder and Piston-Condenser — Air-PumpBarometer and Steam-Gauge-Working BeamParallel Motion- Counter-Sun and Planet Wheel-Fly and other Modes of regulating Velocity-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 reduced, and the wood having been previously jointed with the grain radiating in all directions from the centre, was not liable to expand 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 about two inches 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, and pressing upon it the plate is forced down by screws, which work into the body of the piston. 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. 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 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 the inside of the cylinder, their upper and under sides are also ground perfectly flat, and applied one upon the other. On the On the upper ring is placed a plate of metal, rather smaller in diameter than the cylinder, while a similar flat plate is placed below the under ring, both of which, with the rings between, are attached firmly to each other by means of the piston-rod passing through them.

A shell being thus formed, the rings are each of them cut into three pieces, and in cutting them, such a portion of the metal is taken away as to leave room to introduce between two of the pieces, a spring in form of the letter V, the open end of which is placed outwards, almost close to the circumference; by which means the two pieces against which the two sides of the spring act, are pressed in the direction of the circumference, against the ends of the third piece, so that the three pieces are thus kept so uniformly in contact with the cylinder, that the longer the machine is worked, the better the rings must fit. To prevent