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.

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.