326 WARMING AND VENTILATION. 



mer. It would not be prudent, even in winter, to count upon the heat 

 transmitted by the smoke-pipe to produce a sufficiently powerful draught. 



The sectional area of the chimney should be calculated so that the 

 mean velocity in it may be 5J or 6 feet a second. 



They should open, when possible, in a cupola placed above the center 

 of the hall, which should also receive all the ventilating-pipes of the 

 upper stories. 



A main veutilating-chimney should be constructed of brick and not 

 of metal, abov^e this cupola, the latter to be as low as possible, while 

 the chimney should be made as high as the nature of the building will 

 permit, but at least 20 or 25 feet. 



The sectional area of this chimney should be calculated on the con- 

 dition that the mean velocity of the draught in it should be about 6^ 

 feet a second. 



127. Utilization of the heat given out hy the lights. — In addition to the 

 pipes for carrying oif foul air, the gases produced by the burners of tbe 

 main chandelier or the other chandeliers suspended from the ceiling 

 should also be carried into the cupola. The pipes intended to carry the 

 gases there should be made as small as possible in order not to change 

 the direction of the main draught in the vicinity of these places. 



The diameter of the ventilating-pipe above a chandelier placed near 

 the ceiling and supplied with a metal or glass reflector should be calcu- 

 lated under the condition that it shall only remove 150 cubic feet of air 

 an hour with the velocity of 13 feet a second to a cubic foot of gas con- 

 sumed by the chandelier. 



The single burners in the boxes and corridors should take the air 

 necessary to support the combustion in the interior of the boxes or 

 from the adjoining ventilating-pipes. It will suffice to make their ven- 

 tilating-pipes ^ or f inch in diameter. 



If, as is most probable, the plan of lighting through a glass ceiliug, 

 tried at the Theatre du Cirque and the Lyric Theater, be not adopted, 

 it will be advisable to place a circle of gas-jets at the base of the venti- 

 lating-chimney, to be used only to increase the draught in summer, 

 when the elevation of the temperature of the external air tends to 

 reduce it. It may be assumed that uuder siuiilar conditions each cubic 

 foot of gas consumed will carry off about 800 cubic feet of air. 



A valve should be placed at the foot of the chimney to moderate its 

 draught, and particularly to stop the draught after the close of the play, 

 to prevent useless loss of heat and the entrance of cold air during the 

 night. 



128. Ventilation of the stage. — For theaters where great quantities of 

 smoke are frequently produced duri.ng sham fights, grand illuminations, 

 displays of lire-works, &c., it is well to keep up a strong draught in the 

 upper part of the stage, in order to prevent the gases from being drawn 

 into the auditorium by the draught kept up there, as happened at the 

 Chatelet Theater, where these precautions were not taken. Further- 



