a special hard cast iron, easily drawn when damage or wear necessitates its removal. This liner is secured to the cylinder by bolts E, fig. 5, drawn through the back end of the compression chamber, and the front end, where it enters the cylinder proper, is fitted with a rolling rubber ring F to allow the liner in expanding or contracting to maintain a water-tight joint. The front end of the liner at D1 and D2 is fitted into the bed G, so that in disconnecting the cylinder from the base no water joints are broken.

The piston H is arranged with bosses in the centre through which the pin I is fitted, and by which the connecting rod J is coupled direct to the crank pin K. The base L, to which the bed G is firmly bolted, is provided with an arrangement of baffle plates, to silence the inrush of air through the pipe M, which forms the connection between the base and the box containing the air valve N, arranged below the direct line of fire, with a gas channel O to ensure the gas mingling with the inrushing air. P is the igniting port, in direct communication with the timing valve and hot tube Q. The exhaust port R, fig. 5, is shown

leading to the valve-box S, fig. 6. The cold water is admitted at the extreme bottom end of the combustion chamber at T, and the hot water outlet is arranged at U. V is the cylinder lubricator, from which the oil runs down a small pipe and enters the cylinder at W; it is worked by an endless leather belt from the cross shaft Y, which is driven by the worm wheels at Z.

The cam and lever arrangements are clearly shown. A' represents the air cam, B' the normal working gas cam, C' a gas cam used at starting only, D3 the gas roller controlled by the governor E'; F1 is a cranked rocking shaft, at one end carrying the pin upon which the gas roller works, and at the other transmitting the cam motion to the gas valve through the connecting link G1. The position of the lever H', which has control of the cams B', C', I', and K', is the normal position when the engine is running, but at starting it is moved in the direction indicated by the arrow until the cam I', which is a relieving cam for the exhaust, acts as well as the ordinary cam K' upon the exhausting lever J', and the cam C1 takes the place of the normal gas cam B'; L' is the timing valve cam and M1 a plug for the indicator.

The weak point in this engine is the position of the exhaust valve box. This being placed at the side and in direct communication with the main water jacket C, whilst the main valve opens to the cylinder, the vibration of the engine and the high temperature of the gases tend to break the joint, as the exhaust pipe is at one end rigid and does not respond to any oscillation set up by the engine.

Fig. 8 is an external elevation of a 30 H.P. NOM. engine capable of indicating 100 H.P. with ordinary gas when running at 160 revolutions per minute, and has a cylinder 17 inches diameter with a stroke of 24 inches.

A girder frame type of engine admits of the strains being taken in a direct line between the centres of the crank shaft and the cylinder, and prevents all possibility of the engine' panting.' In this engine there are two main girders, one from each bearing to the cylinder, the crank running between them. The combustion chamber is a separate casting bolted to the end of the cylinder, and the liner of the cylinder is held in position by bolts drawn through it. The air and gas valves are on the extreme

end of the combustion chamber, the exhaust valve at about its centre, in a direct line of fire. A defect in this engine is the angle and direction at which the main bearings are placed, which throws a great proportion of the force of the explosion on the cap bolts, instead of on the main casting; this is not good practice, especially when we consider that even with producer gas a maximum pressure of 320 lbs. per square inch is reached.

This form of engine was also made in coupled twin form, having four bearings, and on the outside of each outer bearing a heavy flywheel, and with cranks arranged so that one piston was full in when the other was full out, the cycle of operations being the same as shown on fig. 2 at C. But as it was impossible for the middle bearings to wear down at the same rate as the outer ones, because of the weight of the flywheels, and the power transmitted, the crank shaft was found to spring backwards and forwards at each throw of the crank; and unless the middle bearings were carefully and periodically lined to the outer ones, the strain would eventually have become too great and produced disastrous results. This evil has been overcome by the tandem engine, having a cylinder on each side of the crank shaft as shown at fig. 9. This type of engine does not, however, divide the work in the best possible manner, as two impulses follow each other at full power, followed by two idle strokes (see fig. 2, as shown at C). In practice, however, this makes very little difference; the advantage of two cylinders over one is that it reduces each individual impulse. Commercial requirements necessitate that sometimes the governor must cut out impulses, and as this covers a large proportion of the time of working, it is the usual condition. This being so, and taking into account the chapter of accidents which occur with the best of governors, it becomes a matter of very little importance if the impulse next to be cut out immediately follow a previous impulse, or has a single stroke between.

These engines are made in the following sizes: 60 H.P. NOM. indicating 200 H.P., and 80 H.P. NOM. indicating 250 H.P., with ordinary town gas, when running at 160 revolutions per minute. If, however, producer gas is used, the I.H.P. is less by about 15 per cent.