The iron smoke-stack which passes through the top story should also be so constructed as to assist the ventilation in the same direction.
This smoke-stack should consist of a cylinder of light boiler iron, and be made smoke-tight to carry off the smoke and surplus gases created by the combustion of the fuel used in firing the kilns.
This cylinder is inclosed within another cylinder made of the same kind of iron, but of sufficiently large diameter to allow of a space of four inches between the two cylinders for the pur pose of providing an air-flue.
The outer cylinder or jacket is pierced with holes one inch in diameter placed six inches apart di'agonally.
When the inner or smoke-stack becomes heated by the fire below it creates a vacuum in the air-flue, and at once the air of the workshop rushes in as indicated by the arrows, and thus passes up and out under the umbrella.
This jacketed and ventilated smoke-stack will also form a perfect protection from fires that might be caused by an over heated flue, because it is impossible to overheat the pierced jacket ; for the hotter the smoke-stack becomes the greater the rush of air into the air-flue, and consequently the greater the ingoing draft of cold moist air.
The elevator-shaft in the centre of the building and the stair ways at either end are open, that is they have no partitions except those high enough to prevent persons falling through or walking off.
By these arrangements all the air as it becomes heated rises toward the kilns and escapes from the building at the roof, taking with it a large portion of the surplus moisture.
Another important feature of this system of ventilation is the method of making the window-sashes. These should be numer ous, and made to slide up and down as is usual in house-sashes, with this exception : only the upper sash ought to have pulleys and weights to allow of it being lowered at will; the lower sash ought to be a fixture, and the sash-frame should reach as near to the ceiling as possible, so as to allow the highest strata of air to pass out easily when the windows are open.
The advantages of such sashes are readily seen. First, the workman can only open the right sash—the top one. Second, there is no draft upon the work or upon the workmen, and yet there is an abundance of fresh air constantly passing through the entire work-rooms. The inlet of air will always take care of itself, provided the builder will furnish the outlet and some motive force to accelerate the motion ; this is done in the above-described method by the radiated heat of the mass of brick-work and the iron chimneys of the kilns, which heat, if not so used, would probably be wasted.
Experience teaches that no matter how perfect the ventila tion, it will not suffice to dry out the surplus moisture quickly enough for profitable working without the aid of some method of auxiliary heating.