shrink or slip off, the engine would continue to run the same as before it broke, as there would be no power to change the valves, since the centrifugal force has only the clutch-back and the centre-weight to lift. The driving power of the governor, when the clutches are in contact, acting on a clutch attached directly to the top of the screw, turns it up, and, acting on a clutch attached to the reversed gear, turns it down. It turns the screw up or down out of clutch before the governor can make a revolution.

The pillow-block boxes are lined with Babbit metal, and are provided with wedge- and draw-screws for the purpose of taking up the wear and lost motion. The wrist- and crank-pins, valveand piston-rods, are made of steel well proportioned and well fitted. The fly-wheels are turned off on the face and sides and are accurately balanced. The Douglas engines are in very general use in the Western States and Territories, and wherever used their reputation for efficiency, durability, and economy has added to their credit.

Technical Terms Applied to Different Parts of

Bonnet.

Steam-Engines.

This term is applied to the covers of the steam-chest. Brasses. - This term is understood to apply to the wrist- and crank-pin, or connecting-rod boxes; but it is used in connection with other arrangements.

Counterbore. A term applied to recesses in the ends of steamcylinders in the clearance space, over which the piston-rings partly travel. The object of the counterbore is to prevent shoulders being formed at each end of the cylinder, which would induce knocking in the engine when any changes are made in the connecting-rod brasses.

Jam-nuts. A term applied to the nuts which lock the adjusting-screws in the piston- and valve-gear of steam-engines; but jam-nuts and lock-nuts are used for many other purposes in connection with the steam-engine.

Pipe-swivel. A long nut containing a right- and left-hand thread. It is used for adjusting the valve-gear of steam-engines, particularly those of the Corliss type; but the pipe-swivel is used for many other purposes than this.

Trunk. A term applied to the hollow tube connected with the pistons of trunk engines in which the connecting-rod oscillates. The term is just as applicable to certain other parts of machinery and arrangements as to the steam-engine.

Trunnions. A term applied to the gudgeons on which the cylinders of oscillating engines vibrate; but it may be, and often is, applied to other machinery as well as oscillating engines.

Terms Formerly Applied to Different Parts of SteamEngines, but which have become Obsolete.

- Gab-lever.-A term formerly applied to an arrangement used for lifting and lowering the eccentric-hook off and on the rockerpin.

Pitman. A term applied to the crank-pins of steam-engines in early times.

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Plug-tree. A primitive valve-gear which superseded the scoggin.

Radius-bar.-A term applied to the connecting-rods of engines in the early days of steam engineering.

Scoggin. This name was given by the boy Potter to the arrangement he invented for opening and closing the valves of steam-engines.

Shackle-bar. This term was used to denote the connectingrod of steam-engines at a period when they were generally made of wood, and strapped with iron at both ends.

Spider. The primitive name for piston-heads of steam-engines.

Questions:

THE ANSWERS TO WHICH WILL BE FOUND IN THE TEXT.

Into what two classes are steam-engines divided, regardless of design, general arrangement, etc.?

Into what two classes are steam-engines in general sub-divided?

Explain the difference between condensing and non-condensing engines, their advantages and disadvantages, and their difference in useful results.

What are the advantages of compound over simple engines, and vice versâ?

State the formulæ for estimating the power of each class of engines.

Explain the difference between automatic cut-off and throt tling engines, and the advantages of the one over the other.

Explain the advantages and disadvantages of the various cutoffs employed on stationary, marine, and locomotive engines.

What are the most valuable features in any steam-engine? What advantages are derived from duplicating the parts of steam-engines?

How would you proceed to fit the crank of a steam-engine on its shaft?

How would you proceed to set up a stationary engine?

How would you proceed to repair a steam-engine?

What is the meaning of the term "dead-centre"? How would you proceed to find it?

Explain the general causes of knocking in steam-engines, and the remedies for the same.

PART THIRD.

Bed-Plates and Housings.

The bed-plate is that part of a steam-engine which forms the connection between the cylinder and the main pillow-block or crank-shaft, and, in many instances, constitutes the support on which they rest. They embrace a great variety of shapes and forms, such as the box side bed-plate, girder-frame, etc., which were all doubtless designed to meet some peculiar requirement, and for each of which special advantages are claimed, the girderframe being in most favor with modern engineers. This is in part due to the fact, that the necessary rigidity can be obtained with less metal than in any other form, and that the strength can be more equally distributed in the line of the strain, and above and below it. Bed-plates are subjected to transverse in addition to tensile and compression strains.

In designing a girder-frame, if it is to be supported only at the ends, due allowance must be made for transverse strains due to the thrust of the connecting-rod. The amount of this strain may be found by dividing the greatest pressure to which the piston may be subjected at mid-stroke by the quotient obtained by dividing the connecting-rod by the crank. Thus, suppose the area of the piston is 200 sq. inches, and it is desired to give ample strength for, say 80 lbs. of steam at mid-stroke; 200 x 80 16,000 lbs., the force on the piston. Then suppose the quotient obtained by dividing the connecting-rod by the crank is 5; 16,000 ÷ 53200 lbs., the pressure on the slides at midstroke. When the engine runs over, that is, when the top of the fly-wheel runs from the cylinder, the weight of the cross-head, and half the weight of the connecting- and piston-rods, must be added to this, and deducted when the motion is in the opposite direction.

When an engine runs under, a support under the frame at the slides (supposing the frame to be of the girder type) would not compensate for weakness of the frame, as the thrust of the connecting-rod being upwards, the upper slide would give, however securely the lower one might be supported. The term housing is applied to the upright frames of both land and marine engines.

The Housing. This term is applied to the upright frame of vertical engines on which the cylinder rests, and which, at its base, contains the main pillow-block bearings.

Steam-Cylinders.

The cylinder is one of the most important as well as the most expensive parts of a steam-engine; it must be made of iron possessing the qualities of hardness and toughness, be moulded and cast with great care, and bored with great accuracy. Cylinders, from the moment they are put into use, have a tendency to wear oblong, also to wear larger in some places than others. This involves the necessity of reboring them, which is one of the largest items of expense incurred in the repairs of a steam-engine.

There are certain peculiarities connected with the wear of steam-cylinders upon which engineers have hitherto been unable to agree, among which is, why the cylinders of different engines of the same size, design, and manufacture, and under the same conditions, wear in opposite directions. The cylinders of some horizontal engines wear more on the lower than on the upper side, while others of the same size and build wear more on the sides opposite the ports, and others on the sides next the ports. Nor is it always the largest cylinders and heaviest pistons that wear most on the lower side of the cylinder. The same peculiarities hold good in relation to vertical engines. On some lines of ocean steamers, where four or five of the engines were built by the same manufacturing firm, and whose design, quality of material, character of workmanship were intended to be as much alike in every respect as it was possible to make them, it was found on exami