SUBSTATIONS. 115 
and intermittent, raising the cost per kilowatt hour by a con- 
sigerable amount. This amount differs greatly according 
to the circumstances of load, but we can get a rough indi- 
cation by making the supposition that the station in 
question is actually delivering power but half of the time 
and on the average at half capacity. 
Under such circumstances the cost per kilowatt hour 
will be, in stations up to 500 k. w. capacity, fifteen to 
twenty per cent in excess of the cost on the basis of a 
twenty hour run at the same proportional load. 
This class of work is that in which the electrical trans- 
mission of power at high voltage to the substations is most 
tempting. ‘The substations on this plan are comparatively 
cheap and the labor item is small, while the cost of copper 
for the transmission line is quite trivial. Nevertheless 
these cases must be very closely scanned, for power trans- 
mission from a steam plant in fairly large units, to compete 
with steam power at cost and economically generated, has 
still a rather narrow margin for profit. 
With this glimpse at the general conditions of economy 
we may profitably pass to concrete consideration of the 
three classes of substation working already mentioned. 
While the West End system already mentioned is the 
best example of auxiliary station practice, it is not yet 
homogeneous, as much of the earlier equipment is still in 
use and the whole system is the result of both agglomera- 
tion and extension. The Charlestown substation is the 
best, as it is also the latest example of its kind and will re- 
pay a little study indetail. Fig. 66 shows an interior view 
of this station and Fig. 67, the plan. The power plant 
consists of two cross compound condensing Allis-Corliss en- 
gines, each forty-eight inches stroke, with cylinders twenty- 
six inches and fifty inches in diameter. Fach engine runs 
at ninety revolutions per minute, is direct coupled to an 
800 k. w. G. E. generator and is provided with a steel fly- 
wheel built up of rolled plates and weighing a little over 
forty tons. As the peripheral velocity of these wheels is 
nearly 6000 ft. per minute, the plate construction is most 
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