Fig. 7 indicates the modification made by Professor Riedler in one of
the Cockerill compressors: a receiver, A, was placed under the two
compressing cylinders, B and C. The first stage is completed in the
large cylinder, B, the air being compressed to about 30 lb. per square
inch; from this it is discharged into the receiver, A, through the
pipe, B¹, where it meets with a spray injection that cools it to the
temperature of the water. The final stage is then effected in the
smaller cylinder, C, which, drawing the air from the receiver through
the pipe, C¹, compresses it to about 90 lb. and delivers it through
the pipe, d, to the mains. We hope shortly to publish drawings of this
compressor in its final form; in its elementary stage Professor
Riedler claims to have obtained some very remarkable results. He says
that the waste spaces in his modification were much smaller than in
the Cockerill compressor, while the efficiency of the apparatus was
largely increased. The actual engine duty per horse power and per hour
was raised, as a maximum, to 384 cubic feet of air at atmospheric
pressure, and compressed to 90 lb. per square inch, a marked increase
on the duty of the compressors in use at the St. Fargeau station. The
Cockerill compressors experimented on at the same time showed a
maximum duty of 306 cubic feet of air. A considerable advantage is
claimed in drawing clean and cool air from the outside of the
building, and beyond the main feature of carrying out the compression
in two stages, Mr. Riedler appears to have shown great skill in
introducing several minor alterations and improvements in the plant.

[Illustration: EFFICIENCY CURVES FOR THREE TYPES OF COMPRESSORS. (Fig.
8, 9, 10)]

Figs. 8, 9 and 10 are diagrams showing the comparative efficiency of