The safety-valve, [Fig. 5], is very important as a safeguard in working. Though they are sometimes omitted, yet safety-valves are essential for security. They allow steam to escape from the boiler when the pressure exceeds a certain amount, and thus the danger of an explosion is removed. The valve illustrated in section has a spiral spring to keep the valve itself on its seat. This is effective when the power of the spring has been definitely gauged, but when the valves are put together haphazard no dependence can be placed on the pressure at which the valve will blow off.
Fig. 5.
The body of the valve is A, shown in section. B is the valve itself, fitted to a rod, D; it rests on the conical seat of A, and is pressed down by the spiral spring within the barrel of A. The body is screwed into the boiler by the thread at the bottom, and the steam coming up the hole C presses on the under side of the valve B. When the pressure of the steam is sufficient to overcome the pressure of the spiral spring the valve is lifted and the steam escapes through the holes F F. The cover E is screwed on the body part and confines the spring; it has a hole through its centre to allow the valve-rod D to pass. Especial attention should always be given to the safety-valve when heat is to be applied to a boiler. See that the valve is not fixed to its seat or in any way confined, or an explosion may follow the want of care.
The engine shown by the illustrations is usually mounted on a three-legged stand, which raises it about two or three inches. A wire stand may be made according to fancy, or perhaps some contrivance may be improvised to support the boiler at a convenient height for applying the heat under it.
A small lamp burning methylated spirits—that is, spirits of wine—will supply the requisite heat. It should have a clean and dry wick of lamp cotton; the size of the flame may be regulated, to an extent, by the amount of wick which is drawn out. The lamp must not be quite filled with spirit—about two-thirds full will be ample—and thus the spirit will not be liable to overflow.
When charging the boiler it is best to use boiling water from a kettle. This will save the time which would be lost in heating cold water with the spirit lamp. The water is poured in through the water-plug hole, A, [Fig. 1]. The boiler must only be filled to a little over half way. The plug is screwed in again and the lamp put under; steam will be generated in due course, and if the fly-wheel is turned in the right direction by hand for a few turns the engine will presently work of its own accord.
It is scarcely necessary to say that the engine above described is of the most simple kind, and every unnecessary detail is omitted. I will now proceed to describe engines of a more elaborate character.