It will be seen that the furnace has at the farther end a door for the removal of ash. As this, if badly fitting, may result in too great a draught, it is well to insist upon good workmanship. Other adjustable air-inlets are provided, and the drum is enclosed in a brick chamber. RATE OF COMBUSTION.--However successful this furnace may have proved in the hands of trained coolies, one must feel that with such a number of air-inlets (whether accidental or designed) there would always be present the possibility of obtaining too rapid a combustion. In the original forms of drums or pot-furnaces of various kinds, a very slow rate of combustion was attained. Naturally a relatively larger proportion of carbon remained unconsumed, and there was a small proportion of ash. In these respects the furnaces resembled charcoal-burners. In point of fact, some estates used this principle for the dual purpose of smoke-curing the rubber, and at the same time obtaining a supply of charcoal to provide fuel for their suction-gas engines. This is a consideration in times when managers are desirous of discovering any devices which tend towards reduction of costs. It will be clear that, under ordinary circumstances, the condition of what remains after the combustion of the fuel gives an indication of the rate at which the wood has burned, and this test should apply to all furnaces. That in which there is the most ash and the least charcoal is the one least to be desired. In direct connection with this consideration, one must recognise that a fire which is sunk below the level of the ground exposes the least surface from which heat may radiate; and hence, in order to obtain the maximum benefit of heat from a slow-combustion furnace, it should be above ground-level, or should have a superstructure from which the heat may be dissipated. Simple drum furnaces, with slow combustion, have the further advantages that a "charge" of fuel will need no attention for possibly eight to ten hours, and practically no ash is found to be ejected. These advantages have great practical importance. The first minimises any disabilities arising from neglect on the part of coolies, and the second makes for increased cleanliness in the drying-chamber. While these advantages would appeal to most estates, there would appear to be a further advantage to small estates which have only temporary timber smoke-houses. With a slow rate of combustion in a furnace of this type, danger from fire is diminished considerably. Bearing in mind the slow rate of combustion, and hence the comparatively low temperature obtained, it will be plain that drum furnaces should be employed in larger number than ordinary open-hearth fires; and the drums can be so placed as to ensure the best possible uniform distribution of heat and smoke.