I have commenced by giving all the preceding detail, in order to show
the groundwork on which I base the estimate of the cost of compressed
air power to consumers, in terms of indicated horse power per annum, as
given in Table II. I may say that, in estimating the engine power and
coal consumption, I have not, as in the original report, made purely
theoretical calculations, but have taken diagrams from engines in actual
use (although of somewhat smaller size than those intended to be
employed), and have worked out the results therefrom. It will, I hope,
be seen that, with all the safeguards we have provided, we may fairly
reckon upon having for sale the stated quantity of air produced by means
of the plant, as estimated, and at the specified annual cost; and that
therefore the statement of cost per indicated horse power per annum may
be fairly relied upon. Thus the cost of compressed air to the consumer,
based upon an _average_ charge of 5d. per 1,000 cubic feet, will vary
from £6 14s. per indicated horse power per annum to £18 13s. 3d.,
according to circumstances and mode of application.

A compressed air motor is an exceedingly simple machine--much simpler
than an ordinary steam engine. But the air may also be used in an
ordinary steam engine; and in this case it can be much simplified in
many details. Very little packing is needed, as there is no nuisance
from gland leakage; the friction is therefore very slight. Pistons and
glands are packed with soapstone, or other self-lubricating packing; and
no oil is required except for bearings, etc. The company will undertake
the periodical inspection and overhauling of engines supplied with their
power, all which is included in the estimates. The total cost to
consumers, with air at an average of 5d. per 1,000 cubic feet, may
therefore be fairly taken as follows:

Min. Max.