of 0.640, and a chemical composition of C5H12, the liquid itself being a mixture of hexane and heptane, the proportions varying with the specific gravity; hexane (C6H14) having a specific gravity of 0.676, and heptane (C7H16) of 0.718.

The composition by weight of petrol having a specific gravity of 0.683 and a boiling-point of 154° Fahr. ishexane, 80 per cent.; heptane, 18 per cent.; and pentane, 2 per cent. The chemical composition is, carbon 83.8 per cent. and hydrogen 16.2 per cent., corresponding to the formula 41 86C6H14 + 6:48C7H16+ C5H12. It requires about 3.5 lbs. of air to consume 1 lb. of petrol, corresponding to a mixture of 100,000 volumes of air to 12.4 volumes of liquid petrol. The density of the vapour from petrol of the above specific gravity and chemical composition is about 3.05, and 1 kilogram of petrol vapour has a volume of 0.254 cubic metre. The proportions of petrol vapour and air, by volume, to give the greatest explosive effect, are therefore

For convenience of reference, we may tabulate the various properties of petrol thus—

The following table shows the specific gravities corresponding to Baumé degrees at 60° Fahr. :—

E

For every eight degrees above 60° Fahr. one degree Baumé should be subtracted from the hydrometer reading, and for every eight degrees below 60° Fahr. one degree is to be added to the Baumé degrees. With a mixture of correct proportions the whole of the vapour will be consumed, and no objectionable odour or fouling of the engine will occur. The limits for the proportions of air and gas for complete combustion are fairly close, hence a carburettor requires to be carefully made and adjusted to ensure a constant supply of correctly proportioned mixture. Experience proves that a mixture of one volume of liquid petrol to about 8380 volumes of air gives good results. With these proportions combustion is rapid and the exhaust clean. Very little odour is to be noticed, and no fouling of the valves and ports. The proportions of the mixture will have to be varied somewhat, according to the quality of the spirit and the state of the atmosphere.

With more than 10,000 volumes of air to 1 of liquid petrol the mixture will not explode properly. Therefore it is advisable to keep the ratio of liquid petrol to air somewhere between 1 to 8000 and 1 to 10,000, which ratios correspond to about 1.9 per cent. and 2.4 per cent. of vapour in each case.

Mixtures are explosive up to a ratio of liquid petrol to air of about 1 to 4000, but when the ratio is reduced to 1 to 3400, the mixture will not be combustible. The ratio of liquid to air of 1 to 4000 is equivalent to about 4 per cent. of vapour, and 1 to 3400 to about 5.5 per cent. At 60° Fahr. air will not be saturated with petrol vapour till it has absorbed about 15 per cent. by volume.

At the present time one hears but little of the surface carburettor, except for motor cycles. The defects of this type were that the lighter constituents of the petrol were apt to be evaporated first, leaving a residue of greater density, and the petrol was splashed about too much. Both of these defects necessitated a constant alteration of the air-valve to maintain the mixture somewhere near its proper proportions. Surface carburettors are more economical of fuel than the jet type, owing probably to the more perfect mixture of the vapour with the air. Also the air usually has considerably more freedom allowed for its passage through a surface carburettor than one of the jet type; hence more power for a given-sized motor can be secured.

Surface carburettors should be so proportioned that the air will pass through them at a speed not exceeding 80 feet per second. In a jet-type carburettor a good suction effect is required, so that the speed of the air may be increased to 100 feet per second.

The faster the air passes the jet, the greater the suction effect will be, so that when the motor increases its speed the proportions of the mixture are apt to be altered. Many devices are in use to prevent the mixture varying in quality, chiefly consisting of extra inlets for admitting air, operated by the suction effect of the piston, when the speed exceeds a predetermined limit. The writer proposes the form of carburettor shown in Fig. 17. The float chamber A is of the usual construction, and requires no