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 the three types of compressors at St. Fargeau--Fig. 10 being a diagram of the Riedler compressor--and indicate the gain derived from the intermediate cooling. The loss is shown to be only 12 per cent., as compared with a loss of 43 per cent. in a large part of the plant, and of 105 per cent. in the earlier compressors of the St. Gothard type. The table given herewith contains a summary of trials made by Professor Gutermuth, and are intended to show the comparative results of an extended trial with three kinds of compressors at St. Fargeau. PERFORMANCES OF COMPRESSORS AT THE ST. FARGEAU CENTRAL STATION.