Station floors for engine and dynamo rooms are, as a rule, constructed of wood. Where very high currents are generated, however, insulated floors of special construction mounted on glass are necessary as a protection from injurious shocks. Brick, concrete, cement, and other substances of a similar nature are objectionable as a floor material for engine and dynamo rooms on account of the grit from them, caused by constant wear, being liable to get into the bearings of the machines.

Where there are no moving parts, however, as in the boiler room, the materials just mentioned possess no disadvantages and are preferable to wood on account of being fireproof.

THEORETICAL DRAFT PRESSURE IN INCHES OF WATER IN A CHIMNEY 100 FEET HIGH
(For other heights the draft varies directly as the height)
Temp. in
Chimney, °F.		 TEMP. OF EXTERNAL AIR. (BAROMETER 30 INCHES)
0° 10°20° 30°40° 50°60° 70°80° 90°100°
200° .453 .419 .384 .353 .321 .292 .263 .234 .209 .182 .157
220 .488.453 .419.388 .355.326 .298.269 .244.217 .192
240 .520.488 .451.421 .388.359 .330.301 .276.250 .225
260 .555.528 .484.453 .420.392 .363.334 .309.282 .257
280 .584.549 .515.482 .451.422 .394.365 .340.313 .288
300 .611.576 .541.511 .478.449 .420.392 .367.340 .315
320 .637.603 .568.538 .505.476 .447.419 .394.367 .342
340 .662.638 .593.563 .530.501 .472.443 .419.392 .367
360 .687.653 .618.588 .555.526 .497.468 .444.417 .392
380 .710.676 .641.611 .578.549 .520.492 .467.440 .415
400 .732.697 .662.632 .598.570 .541.513 .488.461 .436
420 .753.718 .684.653 .620.591 .563.534 .509.482 .457
440 .774.739 .705.674 .641.612 .584.555 .530.503 .478
460 .793.758 .724.694 .660.632 .603.574 .549.522 .497
480 .810.776 .741.710 .678.649 .620.591 .566.540 .515
500 .829.791 .760.730 .697.669 .639.610 .586.559 .534

Chimneys.—These are generally constructed of brick and iron, sometimes of concrete. Iron chimneys cost less than brick chimneys, necessitate less substantial foundations, and are free from the liability of cracking. They must be painted to prevent corrosion, are less substantial, and lose considerably more heat by radiation than do brick chimneys.

Fig. 2,737.—An example of direct connected unit with gas engine power. The view shows a Westinghouse 200 kva., 4,000 volt, three phase, 60 cycle alternator direct connected to a gas engine.