614

HISTORY OF ENGINEERING.

Book I.

are rebated to receive the glass,and the section at the side ofthe opening shows its position;the whole is barred to prevent theprisoner making his escape, at thesame time in sucli a way that thelight is by no means obstructed.In the plan and section of the cellpreviously given, its position andheight from the floor may be seen;as it is placed immediately oppositethe door by which the prisonerenters, which is also provided witha grating, there can be obtained athorough current of air, if it isdesired; the parts marked B andC in the plan of the cell showalso the situation of the fresh andfoul air shafts or pipes obtained inthe thickness of the wall. Theexternal walls, in which these win-dow openings are made, is 1 foot10.^ inches, and that of the wallsbetween the cells is 18 inches inthickness. The whole of these fit-tings arc executed in a most sub-stantial maimer, and the frame isso strongly bedded in the brick-work of the outer wall, that itwould be difficult to disturb ordisplace it.

FT

L

r

PLAN OP CELL WINDOW.

Fig. 57*.

grating, placed near the floor of each cell, on the

The foul air is removed by means ofside next the outer wall, and diagonally opposite the point where the fresh air is introduced ;this grating covers a flue in the outer wall, opening at its upper extremity into a horizontalfoul air-flue iii the roof, communicating with a vertical shaft, raised upwards of 25 feet abovethe ridge. Thus, by the outer air passing

the warming apparatus to the top of each I I

cell, and thence from the floor upwards, I I___J=!>

through the extracting flues and venti-lating shafts, it again passes off into theouter air at a considerable height abovethe building. An uniformity of action isobtained by making each pair of flues usedfor the foul air of equal length on all thestories, and introducing fresh air into them.

Each cell contains 800 cubic feet, and aventilation is obtained of 30 cubic feetper minute, at a cost during the wintermonths of less than a farthing per cell,and during the summer at half thatexpense.

The ascending principle of ventilationin this case extracts the foul air from thecells in consequence of the superior alti-tude of the shaft ; had it been required topass downwards below the floor of thecells into flues in the basement, a power

HANDLE TO ADMIT WARM AIR.

Fig- 579. REGULATORS IN THE WARM AIR FLUES.

must have been produced to overcome the tendency of air to rise when at a high tem-perature ; the ventilation must then have been produced by force, instead of being as it ismerely assisted. From the diffusion which takes place, it is stated that the difference oftemperature at the ceiling or floor of the cell can hardly be detected, and that it seldomexceeds a degree.

The main flues in the roof, for the extraction of foul air, are connected with the verticalshaft, and during the summer a small fire is maintained at the bottom of this shaft, whichraises the temperature of the column of air within it above that of the exterior, or thegeneral temperature of the cells, and thus makes it specifically lighter. In this state it