haust steam; 4, clearance and compression; 5, receiver capacity; 6, piston speed; 7, order of sequence of cranks.

Steam Jackets.-The subject of steam jacketing, and fitting cylinders with working barrels cast separately, has undergone considerable discussion among engineers, although the theoretical advantages of the system have long since been admitted. Keen competition seems evidently to have been one of the reasons why this fitting in many instances has been discontinued, in order to meet the demand for a cheap engine. Thorough investigation of the subject is beyond our present scope; but the action of the cylinder surface, producing, as it does, an influence on coal consumption, deserves more attention than it often receives. In a single cylinder engine with a high ratio of expansion and a large range of temperature, it is well known that in each stroke there is excessive initial condensation, followed by partial re-evaporation. In triple expansion engines, where attention is paid to the equal division of the total range of temperature among the cylinders in which the successive stages of expansion take place, the benefits arising from the use of jackets are naturally not so great as in a single cylinder engine with a high ratio of expansion; but however carefully the triple engine may be designed, the jacketing of at least the intermediate and low pressure cylin

ders is essential to maximum efficiency. Beyond the actual economy resulting from steam jacketing, there are numerous practical advantages of a working barrel cast separately. The cylinder barrel being one of the most vital parts of an engine, it is essential that it should be of the finest material and of uniform quality, in order to prevent it from wearing unevenly, as an oval or uneven cylinder means a leaky piston and increased coal consumption. In the case of a cracked cylinder without an independent liner, the expense of a renewal is very great, while a new liner costs but little; and the additional facility given for heating up cylinders fitted with independent liners is a safeguard agninst accidents of that description.

Cylinder Ratios.-The ratio of cylinder capacities in a triple expansion engine depends on the pressure of steam and the type of engine. In cargo steamers, where economy of fuel is of vital importance and a large range of reserve power is not necessary, the high-pressure cylinder should be of such a diameter that with a cut-off of from 50 to 60 per cent. the theoretical absolute terminal pressure in the low-pressure cylinder shall not exceed 10 lb. per square inch. In war ships, where a large range of power is sometimes required and economy of fuel is not so important, the high-pressure cylinder should be larger in proportion, so that a higher mean

pressure can be obtained. In triple expansion engines on three cranks, the intermediate cylinder should be so proportioned that, with 55 to 65 per cent. cut off, the powers, the ranges of temperature and the initial stresses in the three cylinders may approach equality.

Steam Velocities.-To obtain even approximate equality in powers, temperatures, and stresses, requires the greatest care in designing the steam passages throughout the engine; and unless the velocities of the steam at the various points and the degrees of cut-off by the valves are carefully proportioned, it will be found that these three elements of economy and of efficient working are very far from being realized. The greater number of published results show that the subject is still inadequately appreciated. An expanded high pressure diagram taken from an engine with 135 lb. boiler pressure shows that the velocity in the passages was by no means high, but the pipes were indirect, and owing to specified requirements there were several altogether unnecessary valves; the result was a fall of pressure to 120 lb., or a loss of 15 lb. available initial pressure. When the objectionable bends were removed, the initial pressure in the diagram rose to 130 lb., thus raising the ratio. of expansion and increasing the general efficiency. The stroke of the engine was 3 ft., and the revolutions 104 per minute. A high pressure ex

panded diagram from a cylinder of the same diameter and stroke shows the revolutions to be 70 per minute. This shows how small the initial drop may be, when the sources of loss are thoroughly appreciated.

Piston Valves.-The use of piston valves on the intermediate and low pressure cylinders, especially in engines of moderate power, serves to illustrate how the indirectness of the valve passages impairs the efficiency of the steam, and in some cases more than balances the beneficial effect of reducen friction in the machinery. An expanded diagram taken from the intermediate cylinder of a triple engine fitted with a piston valve shows considerable wire-drawing, which is to be attributed to the indirectness of the valve passages; for in another diagram, taken from a similar engine in every respect, with only a slightly lower speed of steam but with an ordinary slide, the wire drawing is much less.

Low Pressure Cylinders.-The low pressure cylinder is of course the source of the greatest inefficiency; and too much care cannot be taken in the design of the steam ports and exhaust passages. The steam passage should be as short as possible, so as to reduce the clearance to a minimum; and the speed of the entering steam should not be so high as to cause excessive frictional resistance, nor the speed of exhaust so high as to augment the back pressure; conse

quently, the greatest efficiency is obtained when the revolutions and indicated horse power are not required to vary to any great extent. As an illustration, a diagram was taken from a lowpressure cylinder which was designed for 60 to 65 revolutions per minute; on the lightship trial the engines were run at 78 revolutions, and the vacuum was then 134 lb. less than with the same cut off at 68 revolutions, the loss being entirely due to the excessive exhaust velocity consequent on driving the engines at a greater speed than they were designed for. Contracted or indirect exhaust passages in the high pressure and intermediate cylinders have the effect of causing a larger difference between the back pressure on one piston and the initial pressure on the next, thus diminishing the efficiency of the steam. An illustration of this defect is furnished by the diagrams which were taken from a triple expansion engine built in the North of England.

Cut-Off-The cut-off necessary to maintain the equality of the three essentials for an efficient engine is governed to a great extent by the speed of the entering steam and the nature of the passages, inasmuch as the same amount of steam per stroke may enter the cylinder at the ordinary working number of revolutions per minute, as at a higher speed with later cut-off and larger port opening; and in the latter case