The most suitable places for inlets are about seven or eight feet from the floor, and they should open upwards, so that the direction of the incoming air shall be towards the ceiling. The "Tobin" ventilation tubes are constructed on this principle, and seem to be one of the most efficacious. of the various natural systems of ventilation in use.
The following table, which was compiled by the late Charles Hood, shows the cubic feet of foul air extracted by a ventilating shaft of an uniform area of 1 square foot—
| Height of Ventilating Shaft in Feet. |
Excess of Temperature of Air entering the Ventilating Shaft above the External Air. | |||||
| 5° | 10° | 15° | 20° | 25° | 30° | |
| 10 | 116 | 164 | 200 | 235 | 260 | 284 |
| 15 | 142 | 202 | 245 | 284 | 318 | 348 |
| 20 | 164 | 232 | 285 | 330 | 368 | 404 |
| 25 | 184 | 260 | 318 | 368 | 410 | 450 |
| 30 | 201 | 284 | 347 | 403 | 450 | 493 |
| 35 | 218 | 306 | 376 | 436 | 486 | 531 |
| 40 | 235 | 329 | 403 | 465 | 518 | 570 |
| 45 | 248 | 348 | 427 | 493 | 551 | 605 |
| 50 | 260 | 367 | 450 | 518 | 579 | 635 |
As an example of the above, let us suppose a ventilating shaft 30 feet high, arid the difference in temperature of the two airs to be 15°, then the discharge would be 347 cubic feet per minute; if the height be 40 feet, and the difference in temperature 20°, then the discharge would be 465 cubic feet per minute.
A system of mechanical ventilation, combined
