which the cost of the difference of the coal saved (one pound of coal
per hour per horse-power, which would be 1,000 pounds per day) for, say,
three hundred days, three hundred thousand (300,000) pounds, or one
hundred and fifty tons (or six hundred dollars), would pay a fair
interest.

Assuming that the mill owner estimates his capital as worth to him ten
per cent, per annum, then the improvement which would effect the above
mentioned saving must not cost more than six thousand dollars, and so
on. If, instead of being run only ten hours per day, the engine is run
night and day, then the outlay which it would be justifiable to make to
effect a certain saving per hour would be doubled; while, on the other
hand, if an engine is run less than the usual time per day a given
saving per hour would justify a correspondingly less outlay.

It has been found that for grain and other elevators, which are not run
constantly, gas engines, although costing more for the same power,
are cheaper than steam engines for elevating purposes where only
occasionally used.

For this reason it is impossible without considerable investigation to
say what is really the most economical engine to adopt in any particular
case; and as comparatively few users of steam power care to make this
investigation a vast amount of wasteful expenditure results. Although,
however, no absolute rule can be given, we may state that the number
of instances in which an engine which is wasteful of fuel can be used
profitably is exceedingly small. As a rule, in fact, it may generally be
assumed that an engine employed for driving a manufactory of any kind
cannot be of too high a class, the saving effected by the economical
working of such engines in the vast majority of cases enormously