SECTION ON A A  , C and lessening the chimney waste, actually increase the efficiency even if at the same time it is accompanied by a slight incompleteness of combustion . Mechanical Stoking.—Most boilers are hand-fired, a system involving much labour and frequent openings of the furnace doors, whereby large quantities of cold air are admitted above the fires . Many systems of mechanical stoking have been tried, but none has been found free from objections . That most usually employed is known as the " chain-grate " stoker . In this system, which is illustrated in fig . 13 (Woodeson boiler), the grate consists of a wide endless chain formed of short cast-iron bars; this passes over suitable drums at the front and back of the boiler, by the slow rotation of which the grate travels very slowly from front to back . The coal, which is broken small, is fed from a hopper over the whole width of the grate, the thickness of the fire being regulated by a door which can be raised or lowered as desired . Thus the volatile portions of the coal are distilled at the front of the fire, and pass over the incandescent fuel at the back end . The speed of travel is so regulated that by the time the remaining parts of the fuel reach the back end the combustion is nearly complete . It will be seen that the fire becomes thinner towards the back, and too much air is prevented from entering the thin portion by means of vanes actuated from the front of the boiler . Draught.—In most boilers the draught necessary for combustion is " natural," i.e. produced by a chimney . For marinecombustion of about 15 to 20 lb. of coal per sq. ft. of grate area per hour can be obtained .

With forced draught much greater rates can be maintained, ranging from 20 lb to 35 lb in the larger vessels with a moderate air pressure, to as much as 70 and even 8o lb per sq. ft. in the

express types of boiler used in torpedo boats and similar craft . Performance of Boilers.—The makers of several types of boilers have published particulars regarding the efficiency of the boilers they construct, but naturally these results have been obtained under the most favourable circumstances which may not always represent the conditions of ordinary working . The following table of actual results of marine boiler trials, made at the instance of the British admiralty, is particularly useful becuase the trials were made with great care under working conditions, the whole of the coal being weighed and the feed-water measured throughout the trials by skilled observers . The various trials can be compared amongst themselves as South Welsh coal of excellent quality was used in all cases . In experimental tests such as those above referred to, many conditions have to be taken into account, the principal being the duration of the trial . It is essential that the condition of the boiler at the conclusion of the test should be precisely the same as at the commencement, both as regards the quantity of unconsumed coals on the fire-grate and the quahtity of water and the steam-pressure in the boiler . The longer the period over which the observations are taken the less is the influence of errors Duration coal ion o in the estimation of these particulars . Further, in order properly to represent working conditions, the rate of combustion of the fuel throughout the trial must be the same as that intended to be used in ordinary working, and the duration of the test must be sufficient to include proportionately as much cleaning of fires as would occur under the normal working conditions . The tests should always be made with the kind of coal intended to be generally used, and the records should include a test of the calorific value of a sample of the fuel carefully selected so as fairly Air f thechimney Water Evaporated evapor- Efficiency be conveniently treated together, because similar materials and methods are employed in each, notwithstanding that many points of divergence in practice generally relegate them to separate departments . The materials used are chiefly iron and steel . The methods mostly adopted are those involved in the working of plates and rolled sections, which vastly predominate over the bars and rods used chiefly in the smithy . But there are numerous differences in methods of construction .