the boiler from damage by overheating, other parts of the engine might be damaged if the fire dribbles burning meths. Some engines may be fitted with a pump driven off the motion or the axle, in which case a by-pass valve allows the driver to balance the water input against consumption, but where only a hand pump is fitted, the driver must learn the intervals at which it is prudent to replenish his boiler. It follows that although Smithies boilers will run unattended for a while, it is necessary to keep an eye on them all the time, as performance depends on the balance of fire and water level. In short, the Smithies is a simplified version of the 'proper' loco boiler and demands the same attention.

The true loco boiler is our third type, and the rarest on Gauge 0 and Gauge 1. Here, the boiler shell is a water drum, through which pass a number of flues. The greater part of the water-boiling activity takes place in the immediate vicinity of the firebox, which is surrounded by water spaces. Like the Smithies, the true loco boiler fire needs constant urging and the water level has to be more carefully maintained-it must never be allowed to go dry. It is very complicated and expensive to make. It has, however, several advantages possessed by no other model boiler. First, it is the only miniature suitable for coal firing. Second, it is without doubt the Real Thing. Third, it demands great skill of the man doubling as driver and fireman, and if he can produce that skill, he knows that at the throttle of his coal fired true-loco-boilered engine, he is king of the road at any assembly of steam garden railway men.

From boilers we progress logically to cylinders and valve gears. Cylinders are either oscillating or reciprocating. The former type has no valve gear as such, for the waggling of the cylinder brings the inlet and exhaust ports into play at precisely the right moment, while the piston rod is also the connecting rod. They are thus relatively easy and cheap to make, but unless the piston is fitted with a neoprene ring, it tends to wear the lower part of the cylinder oval. The neatest way to reverse it is to employ, as on the Mamod SL 1, a rotary valve that reverses the steam flow-when going backward the

In its natural habitat. Archangel's 'Sgt Mur-phy' class Empress of Blanding on the weekly goods on David Rowland's Aiderbrook Valley Railway.
(photo Peter Dobson)

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