110. The carrying capacity of a flue or duct is controlled by the temperature of the hot air and the height of the flue, in the same manner as a chimney. A flue which extends to the third story of a building will discharge more air per minute than a similar flue which discharges at the second or the first floor. This is because the column of hot air extending to the third floor is higher than the others, and consequently the upward pressure is greater. In Table 17, which gives the velocity of air in flues, in feet per minute under natural draft, an allowance of 50 per cent. over the theoretical flow has been made for friction and other common resistances. This will be sufficient for all ordinary circumstances. It should be borne in mind that the volume of flow shown in the table cannot be attained unless the air in the room is permitted to escape freely and as rapidly as the fresh warm air is inclined to come in.

## Table 17

 Difference inTemperatureDegrees P. Height of Flue in Feet. 10 15 20 30 40 50 60 80 100 10 108 133 153 188 217 242 264 306 342 15 133 162 188 230 265 297 325 375 420 20 153 188 217 265 306 342 373 435 485 25 171 210 242 297 342 383 420 485 530 30 188 230 265 325 375 419 461 530 594 40 216 265 305 374 431 482 529 608 680 50 242 297 342 419 484 541 594 680 768 6O 260 327 376 460 532 595 650 747 842 70 288 354 407 498 576 644 703 809 910 80 308 379 435 533 616 688 751 866 972 90 326 401 460 565 652 728 795 918 1029 100 342 419 484 593 684 765 835 965 1080 125 384 470 541 604 766 857 939 1085 1216 150 419 514 593 726 838 937 1028 1185 1325

The difference in temperature given in the table is that existing between the outer atmosphere and the average of the air in the flue.

Each register should be supplied by an independent vertical duct.