In designing the outside cell type of prison the problem is one of providing a certain definite external wall area for the sides of the prison, since for given dimensions of cells and a specified number of these cells a definite external wall area is required. The problem of maximum economy in construction then resolves itself into a question of providing the maximum of wall area with the minimum of floor area. Two general forms of outside cell buildings have been proposed: one, the narrow rectangular type adopted at the Kilby prison, and the other the cylindric type. It is demonstrated in geometry that of all figures a circle has a maximum of area for a given length of periphery, while a very narrow rectangle or quadrilateral has a minimum of area for a given periphery. It is, therefore, obvious that for a given external wall area, or a given cell capacity, the narrow rectangular type is more economical in first cost, since it reduces the ceiling and floor area to a minimum. Economy in construction was also promoted by constructing our cell house five tiers high instead of four tiers, as is more usual. Finally, additional economy was secured by the use of the multiple man cell. Our multiple man cells accommodate six prisoners, while three single man cells of the same cubic contents containing two partitions, two extra prison doors and locks, two extra lavatories, two extra water-closets, two extra radiators, with all of the necessary connections to these utilities, will only accommodate half as many prisoners. It will, therefore, cost more than twice as much in cell-house construction to incarcerate a given number of prisoners in the single man cells than in the six man cells. It will, therefore, be noted that we have secured economy in cell-house construction (which is the most expensive item of prison construction) by increasing the density of occupancy in the cell houses. But this density of occupancy carries with it responsibilities in the matter of providing adequate ventilation for the inmates. Recognizing this responsibility, we have designed our cell house to secure the very maximum of natural ventilation. This is secured, first of all, by a very large proportion of window area to wall area; by ventilating the windows top and bottom; by constructing the cell house with a cross-section shaped like a chimney, with a large number of large ventilators on top of an open pitched roof, so as to secure the very maximum of chimney effect and also the very maximum effect from breezes.