relative positions of the engine and boiler houses are indicated in
Figs. 2 to 5, where F shows the end of one group of boilers; the air
supply for the compressors is led from the central raised portion, S,
of the roof.

Professor Riedler's first experiments in improving the efficiency of
air compressors were made with one of the Cockerill compressors in use
at the St. Fargeau Station, and considerable difficulty attended this
work, because the machinery was necessarily kept almost in constant
operation. These compressors were designed by MM. Dubois and Francois,
of Seraing. Two of their leading features were the delivery of the
compressed air at as low a temperature as possible, and with a
relatively high piston speed of about 400 ft. a minute. The former
object is attained by the injection of a very fine water spray at each
end of the air cylinder, and its rapid removal with each stroke; the
free as well as the compressed air flows through the same passages,
one at each end of the cylinder; the inlet valves being placed at the
side of these passages, and the outlet or compressed air valves at the
top, the compressed air, entering a chamber above the cylinder, common
to both valves, and passing thence to the reservoir. The compressed
air valves, which are seven in. in diameter, are brought back sharply
to their seats at each stroke, by a small piston operated by
compressed air flowing through a by-pass from the chamber. The
illustrations published by us on page 686 of our forty-seventh volume
show the construction of these compressors. The engravings on page 683
of the same volume illustrate the compressors used in a somewhat older
part of the installation; they were made by M. Blanchod, of Vevey, and
a passing reference may be made to them. The air is admitted through
valves in the cylinder, and is forced out through spring-loaded
valves; water is admitted into the cylinder to cool the air.