The results thus obtained were so satisfactory that the designs were
prepared for the great compressors to be operated at the new central
station on the Quai de la Gare by the 2,000 horse power engines.

The transmission of the compressed air through the mains is
unavoidably attended with a certain percentage of loss, which, of
course, increases with the length of the transmission, the presence of
leakage at the joints, etc. Professor Riedler has devoted considerable
time to the investigation of this source of waste, and we shall
presently refer to the results he has recorded; in the first place,
however, we propose to consider what he has to say on the subject of
utilizing the air at the points of delivery, and the means employed
for obtaining a relatively high efficiency of the motor.

In the earliest stages of the Popp system in Paris it was recognized
that no good results could be obtained if the air were allowed to
expand direct into the motor; not only did the formation of ice due to
the expansion of the air rapidly accumulate and choke the exhaust, but
the percentage of useful work obtained, compared with that put into
the air at the central station, was so small as to render commercial
results hopeless. The practice of heating the air before admitting it
to the motor is quite old, but until a few years ago it never seems to
have been properly carried out; in several mining installations where
this motive power had been long used, more or less imperfect attempts
had been made to heat the air; in one instance only, recorded by
Professor Riedler, was an efficient means employed. In this case a
spray of boiling water was injected into the cylinder and mixed with
the air at each stroke, with the result that a very marked economy was
obtained.