purchase. In these days of high steam pressures, a well constructed
non-condensing engine can, no doubt, be made to approximate closely to
the economy of a condensing engine, but in such a case the extra cost of
the stronger boiler required will go far to balance the additional cost
of the condensing engine.

Having decided on the form, the next question is, what "class" of engine
shall it be; and by the term class I mean the relative excellence of the
engine as a power-producing machine. An automatic engine costs more than
a plain slide-valve engine, but it will depend upon the cost of fuel at
the location where the engine is to be placed, and the number of hours
per day it is kept running, to decide which class of machine can be
adopted with the greatest economy to the proprietor. The cost of
lubricating materials, fuel, repairs, and percentage of cost to be put
aside for depreciation, will be less in case of the high-class than in
the low-class engine, while the former will also require less boiler
power.

Against these advantages are to be set the greater first cost of the
automatic engine, and the consequent annual charge due to capital sunk.
These several items should all be fairly estimated when an engine is
to be bought, and the kind chosen accordingly. Let us take the item of
fuel, for instance, and let us suppose this fuel to cost four dollars
per ton at the place where the engine is run. Suppose the engine to be
capable of developing one hundred horse-power, and that it consumes five
pounds of coal per hour per horse-power, and runs ten hours per day:
this would necessitate the supply of two and one-half tons per day at
a cost of ten dollars per day. To be really economical, therefore, any
improvement which would effect a saving of one pound of coal per hour
per horse-power must not cost a greater sum per horse-power than that on