The cooling water used was 18.73 lbs. per minute, with a rise in the temperature of 40° Fahrenheit.

So-called "gasoline" engines are neither properly gas nor oil engines, and come into consideration in Chapter XXXII., as they use vapour of a highly explosive character.

CHAPTER XXXI.

THE POWER OF THE EXPLOSION OF VAPORIZED MINERAL OIL IN THE OIL-ENGINE.

It is rapidly becoming apparent that in the vaporization and explosion of mineral oil we have before us the most serious competition with steam, or indeed with any other form of "heat-engine." The long-continued labours of Priestman have resulted in perfecting the means of vaporization of crude and low-grade oil to such a degree that the early difficulties experienced in the process have disappeared, and now a number of inventions, differing only more or less in detail, have entered the field.

From the point of view of the user of power it is a good thing that so much attention is being devoted to this subject, for there can be no doubt that a form of motive power, less complicated by outside considerations than steam, is widely needed, and, while the gas-engine is an excellent substitute under certain circumstances of gas-supply, it is either confined to such localities, or needs a gas-making apparatus demanding nearly the same attention as a steamboiler.

In the oil-engine we have, even in its present stage, which will by no means represent its perfected forms, a power at once independent, needing little attendance and moderately flexible, while the basis of supply for its necessary material is world-wide, and the visible supply tends to increase. The class of machine described in this chapter must not be confounded with the "gasoline" or "vapour " engine which utilizes a volatile and highly inflammable liquid, and is dealt with in the succeeding chapter,

Oil Supply. The cost of mineral oil at present varies very widely in different countries owing to costs of carriage, short-sighted monopolies, and the action of syndicates, such as that which is doing such harm to the development of industry in Spain.

The cost is to some extent contingent on the method of transport of the material. In the United States of America long lines of pipes have been laid to sea-ports and large towns. Oil is also transported in bulk in vessels' holds and in special tank-cars on railroads. In barrels the resale of an empty 40-gallon barrel will average 3s. 4d., reducing the cost of oil by about 1d. per gallon. Arrangements as to the cheap delivery of the oil should be examined, and if possible made, previous to the purchase of one of these useful engines.

In the case of the larger powers a storage should be provided, preferably in iron tanks, and where several users of power are located in the same place such a storage might profitably be divided between them.

Agriculturists in England are rapidly grasping the advantages of this power for farm purposes in the portable form in which the enterprise of several manufacturers is producing it. The carriage in country districts of the quantity of oil representing a given power is found to be so much less than that due to other fuels that the oil-engine will inevitably displace the steam portable engine to a very large extent when its details and management have become more widely understood. At present it is rather looked upon as a complicated form of the gas-engine, which is a false impression, although it is in many respects very similar to that now well-known machine.

The Operation of the Oil-Engine.—The oil-engine generally consists of the usual cylinder, the work in which is usually confined to one side of the piston, as has been found convenient in most gas-engines. The valve gear, driven by

one or other form of gearing from the crank-shaft, operates · admission valves, and also a very small pump which supplies oil to the vaporizer. This apparatus, which is the essential feature, is the subject of a number of ingenious devices and patents. In it the oil is subjected to such a degree of heat, when finely divided in a spray, as completely converts it into a vapour, which is then treated as gas in the gas-engine, and is exploded when mixed with air at a given density or compression.

A lamp is in some cases used to heat the vaporizer when first starting, and this is urged by a small hand-fan or blower generally supplied with the machine. When the engine has made a few turns the heat derived from the explosions is sufficient, unless working at a small proportion of its capacity, to maintain a due temperature in the vaporizing chamber, and the lamp may then be extinguished.

Starting. The oil-engine is subject to the same disadvantage as the gas-engine in requiring manual force to turn it over its dead-centres at first to give it the necessary start.

This, however, in small powers is not a great matter, and self-starting apparatuses of the same kind as those applied to some of the larger gas-engines may be arranged to work in connection with the oil-engine.

But so long as the supply of oil lasts the machine requires no other attention beyond lubrication, and the leading makers give amply sufficient guarantees of workmanship and material.

The "Cycle.”—The "cycle," or recurrence of operations in the cylinder, is now almost universally that known as the four-cycle, by which an impulse is given, at maximum power, every other revolution of the crank-shaft. The number of impulses at full, three-quarter, and half power, thus obtained are to be seen in the diagram of "cycles" in the previous chapter.

Water Circulation.—A supply of cooling water is a neces

sity, and in the portable form it is conveniently arranged in a tank carried under the machine, but when thus situated below the level of the cylinder a circulating-pump is needed as part of the apparatus.

In the fixed form of engine a natural circulation can be obtained by the use of a tank fixed at a proper level, or by a running supply. The cost of freight upon such a tank may, however, more than outweigh that of a circulating pump.

Adaptations. The engine is generally constructed upon a hollow base, in which a supply of as much as a week's consumption of oil may be stored, rendering it so self-contained, as well as solid, that very little fixing to foundations is required, it being sufficient, with sizes up to five effective horse-power, to secure it to a wooden floor by coach-screws.

Thus, where fuel of any kind is dear and water is scarce, the oil-engine offers peculiar advantages, especially now that the portable form has become obtainable, in which the whole of the water and oil-supply for a week's running may be transported. Compared with steam, the absence of attendance, and of fire and sparks, together with the general cleanliness of the apparatus, will prove in many cases of great advantage. For pumping in mine headings a number have been employed successfully, and even for rock-boring with a rotary cutter, while others have been applied to hauling purposes and to compressing air for fog-signal stations.

Their use for providing the power for domestic installations of the electric light is a greatly increasing one in Great Britain, and for general agricultural purposes, dairy, sawing, crushing, and mill-driving, they seem to prove quite as satisfactory as any motor.

The various classes of fixed and portable oil-engines on the market, up to the spring of 1894, were neatly classified by the Engineer newspaper in its reports of the trials of these machines at Cambridge, which, somewhat modified for the sake of greater clearness, stand as follows: