When the building under construction is of considerable size a portable engine becomes one of the first necessities, and sometimes two or more of these engines can be employed to advantage on different processes about the work. The concrete mixer, the friction hoist, the mortar mill, the stone or brick crusher, and various other machines in use on the site all require power to drive them, and this can be supplied by the portable engine more economically, perhaps, than from any other source. Where electric current can be obtained from supply company's mains close by, the convenience attendant upon its use for power purposes may often induce a contractor to install electric motors in preference. A point to be borne in mind, however, in this connection is that, in London, many of the electric supply companies do not use the same voltage or pressure in their mains. Thus an electric motor which is suitable for use in one part of London may, and probably will, be quite useless in another district. No restriction of this kind applies to the portable engine, as coal, oil, and water can always be obtained locally with ease.

The engine itself consists of an ordinary horizontal steam engine, with either one or two high-pressure cylinders, or in the larger sizes with a high and low-pressure cylinder, this latter type being known as a compound engine. The cylinder or cylinders are firmly bolted to the top of the fire-box of the locomotive boiler which supplies the steam, and the crank shaft is carried either by cast-iron horns containing the bearings, or preferably by boiler-plate brackets riveted on to the fore-end of the boiler barrel, on which brackets the ordinary bearings or plummer blocks are bolted. In the latter arrangement it is desirable that the plummer-blocks should be connected to the cylinder casting by a strong wrought-iron stay, which will take all the thrust and pull of the reciprocating motion of the engine, thus avoiding all unnecessary strain on the plates or shell of the boiler. A fly-wheel, which also serves as a driving pulley, is mounted on one end of the crank shaft, and it should always be stipulated in ordering one of these engines that the crank shaft shall be sufficiently long to take a fly-wheel or pulley on either or both ends. The exhaust or waste steam from the cylinder is generally conveyed by a pipe of large bore along the top of the boiler barrel to the chimney, so that a good draught for the furnace is obtained by the slight vacuum caused by the steam blowing up it. The chimney should be hinged near the base, and arranged to fold back on to a forked rest provided for it for convenience during transport. A small pump for supplying water to the boiler is fixed on one side of the barrel of the boiler, motion being given to it by an eccentric and rod on the crank shaft. This pump should be of such size that when the engine is running continuously at its maximum power the boiler is kept fully supplied It is, however, wise to have a further water supply to the boiler provided for, by having an injector fitted. By means of this arrangement the engine can be kept working safely should anything happen to cause the pump to cease its supply.-a not infrequent occurrence when the water being pumped is dirty or contains scraps of wood, sand, etc.

The boiler of the portable engine is almost invariably of the locomotive type, i.e. the furnace is containedI in a rectangular "fire-box" surrounded on all sides by a water-jacket, and the products of combustion, flames, smoke, etc., are led from the fire-box through a large number of steel tubes of small diameter to the chimney. These tubes, which run from end to end of the barrel of the boiler, are of course submerged in the water, and, being of small diameter and numerous, they expose a very large surface, heated by the fire, to the water. Steam is generated in this type of boiler very rapidly, a consideration which must be taken into account when it is stated that it is not the most economical type as regards fuel consumption.

The essential point which constitutes the "portable" engine is the fact that it is mounted on wheels, either of wood or preferably wrought iron, and provided with shafts for horses, so that it can be drawn from place to place. When placed in position for work these wheels should be fixed in place as strongly as possible by inserting large wooden wedges back and front of them, otherwise the whole engine will tend to sway backwards and forwards with the reciprocating motion of the piston. When working near other buildings or any inflammable material the chimney of the engine should be provided with a wire cage or spark arrester, as the force of the blast of the exhaust steam will often carry small live coals out of the top and throw them a considerable distance if this simple precaution is neglected.

Portable Engines 278

Fig. 246.

Machinery in Yard The Cross-Cutting Machine illustrated in Fig 246 is adapted to the manufacture of doors, sashes, and other joinery. It consists of a strong, yet light, iron frame, swinging freely on a counter shaft, and carrying at its lower extremity a saw spindle. The saw and frame, which are counterbalanced, are drawn by a handle across the wood to be cut, as shown. In some cases it is found more convenient to swing the frame from a counter shaft below. An iron bench, with friction rollers, fence, and gauge can be arranged for carrying the timber, and a safety guard should be placed over the saw.

Self-Acting Saw Benches (Fig. 247) are adapted for cutting planks, deals, and battens into boards or scantlings, at a speed up to 60 feet per minute. They are made in one casting, with a steel spindle running in gun-metal bearings, and have fast and loose pulleys on the outside, the end of the spindle being carried by a strong swan-neck bracket bolted to the bench. The top is planed and polished, and fitted with a parallel fence, with lever and pressure rollers for keeping the timber to the fence. A self-acting motion, with drag rope having variable rates of feed, for drawing forward the timber, is attached to the bench, which can have two carriages running on rails (one at each end), for cutting logs or long scantlings, one of which is shown in the illustration.