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III] BOILER IMPROVEMENTS 51


which material is also used as a covering for the lower portions of the water-tubes and collectors. The front tubes are disposed so as to form a horizontal flue for the passage of the products of combustion to the smoke-box, the tubes being arranged in rows in a longitudinal direction. As in the case of the back group, escape of gases from the sides is prevented by close contact of the tubes in the two outer rows.

The tubes connecting the drum and collectors are inlet on the underside of the drum, and a very low level of water suffices to cover them entirely. This is held to constitute an important advantage peculiar to this type of boiler, as the volume of free water comprised within the maximum and minimum levels is sensibly greater than that which is available within the same limits in cylindrical boilers of the ordinary smoke-tube type. This means a greater reserve of energy, which can be drawn upon when long gradients have to be negotiated.

The surface of ebullition remains nearly constant whatever may be the height of the water-level in the drum, because it is always in the neighbourhood of the horizontal diameter of the drum. Such is not the case in boilers of the ordinary type, in which the sur- face of cbullition diminishes progressively with the height of water-level, so that priming, which results from such a diminution of evaporating surface, cannot take place in the new type of boiler. The outside4-2

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