To derive the full benefit of natural heat, the roof is covered with felt or with sheet-iron, so that in fine weather the temperature of the air in the upper floor is raised considerably by contact with the hot roof, and its capacity for absorbing moisture much increased, preparatory to its being drawn down through the mass of coffee in the upper floor. Even in the cool climate of the district of Upper Hewahette, at an elevation of 4500 feet, in a fine day the temperature of the air under a felt roof is 120° when the fans are not working, so that a great drying power is thus made available at no expense.
The lower floor, on the other hand, is adapted for the application of artificial heat for the purpose of evaporating the surface water from each separate batch of coffee as it is taken from the washing cisterns preparatory to its being deposited in the upper floor. In wet weather this is essential, for the atmospheric air being then saturated with moisture, no drying can take place until its capacity for absorbing moisture is increased by an increase of temperature. One other reason for adopting this arrangement is, that when the coffee is first taken wet from the washing cisterns the interstices of the beans are more or less occupied with water, and thus present a medium less pervious to air than is the case when the surface water has been dried off. Consequently it is desirable that, until this has been done, the depth of the coffee should not exceed six inches, and, to be equal to every emergency, the heating power ought to be sufficient in the wettest weather to evaporate the surface water from the produce of a day’s picking (within twenty-four hours), so as to allow of its being removed into the upper floor. The daily number of bushels picked from any, or the same plantation, is of course a variable quantity: depending on the extent of the cultivation, the quality of the trees, the number of hands employed, and the elevation of the land; the latter, when considerable, having the effect of prolonging the picking season. The stove is more than sufficient for a daily picking of 400 bushels of cherries.
The heating stove is square, has a waggon head with a semicircular opening in the centre for the passage of air, and is constructed of stout sheet-iron. It is placed within an arch, with a clearance of nine inches all round also for the passage of air, the guiding principle in its construction being to adapt it to the burning of wood, and to expose as much heating surface as possible to the air which flows past it into the air-chamber beneath the ground-floor. The stove opening is the only one which admits air to the coffee on the ground-floor. Consequently, when it is more or less closed by a damper, the power of the fans is exerted either in part or altogether on the mass of coffee in the upper floor.