v is a safety chamber of wire gauze to prevent the flame from the combustion chamber reaching the carburetter.
The construction of the twin tap will be seen from fig. 8. It consists of two concentric cylinders; the outer cylinder is in one piece, but the inner one is separated into two short cylinders, r and r', which are manipulated inside the outer one by the levers o and o'.
A is the upper part of the carburetter, corresponding to c in fig. 7. b, is the aperture through which the crude vapour ascends into the twin tap, passing through the wire gauze c en route. d is an aperture on top of the twin tap, through which the ordinary air enters. By the suction of the engine this crude vapour and air are drawn into r', and, mixing as they go, enter r, and so into the pipe e, and thence are drawn into the combustion chamber of the engine. As already mentioned, the lever g' operates the cylinder r', through which circular holes are cut, to admit the crude vapour and air. When the lever is in a certain position nothing but crude vapour is admitted to r'. By operating it, however, the air inlet d is gradually opened, and at the same time the vapour inlet b is gradually shut, thus regulating the quality of the mixture, or in other words the relative proportions of air and crude vapour. Similarly the lever g, operating the cylinder r, either opens or closes the aperture f, thus regulating the quantity of mixture which passes from the mixing chamber to the combustion chamber.
Elsewhere in this volume the importance of having the mixture correct both in quality and quantity will be duly dealt with.
The sectional view of that portion of the tap which governs the quality of the mixture, shown in figs. 9 and 10, will perhaps more clearly explain the way it works. In fig. 9 it will be seen that the crude gas aperture b is almost wholly closed, while the air aperture d is almost fully open. In fig. 10 b is half open and d half shut. It can be seen how the tap can be operated so that b will be wholly closed and d wholly open, or vice
