construction of the motors and the treatment of the air at the point
of delivery into the engine.

A large number of motors in use among the subscribers to the
Compressed Air Company, of Paris, are rotary engines developing one
horse power and less, and these in the early times of the industry
were extravagant in their consumption, to a very high degree. To some
extent this condition of things has been improved, chiefly by the
addition of better regulating valves to control the air admission.

As altered, the two horse power rotary motors, when employed as cold
air engines, a method often desired in special industries, consume
1,059 cubic feet per hour and per indicated horse power; with a
moderate degree of heating, say to 50 deg. Cent., this consumption
falls to 847 cubic feet. The efficiency of this type of rotary motors
with air heated to 50 deg. may now be assumed at 43 per cent., not a
very economical result, it is true, and one that may be largely
improved, yet it is evident that with such an efficiency the use of
small motors in many industries becomes possible, while in cases where
it is necessary to have a constant supply of cold air, economy ceases
to be a matter of the first importance.

Some useful results were obtained with compressed air used in crank
engines; it is to be regretted that with this, also, apologies have to
be made for the imperfect design and construction; they were old steam
engines, some of those of two horse power losing from 25 to 30 per
cent. by their own friction; some of the others tried, however, were
far better, a newer type losing only from 8 to 10 per cent., while the
80 horse power referred to below showed an efficiency of 91 per cent.
From these trials Prof. Riedler deduces--assuming 85 per cent.