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.

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.

--------------+-------+--------+------+-------+--------+--------+----------+

| | | | | | | |

|Revolu-| Horse- | |Amount |Quantity| Cubic | |

Compressors. | tions | Power |Effic-|of Air | of Air |Feet of |Final Air |

|of Eng-|Absorbed|iency.|Passing| Passing|Air per |Pressure. |

|ine per| by | |through| through| Horse- | |

|Minute.|Compres-| | Inlet | Valves | Power | |

| | sors. | | Valves| per | and per| |

| | | | each | Hour. | Hour. | |

| | | |Revolu-| | | |

| | | | tion. | | | |

--------------+-------+--------+------+-------+--------+--------+----------+

| | | | cubic | cubic | |lb. per |

1. | | | | feet | feet | |sq. in. | Sturgeon | | | | | | | | Compressor | 37 | 302 | .87 | 41.67 | 91,507| 261.3 | 90 |

Diameter of | 37 | 258 | .87 | 38.13 | 84,650| 276.1 | 90 |

cylinder, | | | | | | | |

23.62 in. | | | | | | | |

and 21.66 in.;| | | | | | | |

stroke, | | | | | | | |

48.63 in. | | | | | | | |

| | | | | | | |

2. | | | | | | | | Cockerill | 40 | 337 | .83 | 46.61 | 111,864| 281.83 | 90 | Compressor. | 45 | 353 | .83 | 46.61 | 125,844| 302.66 | 90 |

Diameter of | 40 | 342 | .88 | 49.43 | 118,632| 296.65 | 90 |

cylinder, | 46 | 377 | .85 | 48.02 | 132,534| 298.77 | 90 |

25.98 in.; | 38.67 | 324 | .89 | 50.14 | 116,434| 306.19 | 90 |

stroke, | 38.5 | 337 | .89 | 50.14 | 115,818| 294.18 | 90 |

47.24 in. | 38.6 | 329 | .91 | 50.84 | 117,740| 305.13 | 90 |

| | | | | | | |

| | | | | | | |

3. | | | | | | | | Riedler | 52 | 615 | .985 | 77.34 | 241,300| 353.50 | 90 | Compressor. | 60 | 709 | .985 | 76.98 | 277,128| 353.50 | 90 |

Diameter of | 38 | 422 | .985 | 77.34 | 176,330| 376.12 | 90 |

low-pressure | 39 | 424 | .985 | 77.34 | 181,030| 384.60 | 90 |

cylinder, | | | | | | | |

42.91 in.; | | | | | | | |

diameter of | | | | | | | |

high-pressure | | | | | | | |

cylinder, | | | | | | | |

26.38 in.; | | | | | | | |

stroke, | | | | | | | |

47.24 in. | | | | | | | |

--------------+-------+--------+------+-------+--------+--------+----------+ 

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.

After a number of experiments, Mr. Popp arrived at the conclusion that the simplest mode of heating, if not the most efficient, was at all events the most suitable, as it was a matter of the first importance that subscribers should not be troubled with the charge of any apparatus involving complication or careful management; he therefore adopted a simple form of cast iron stove lined with fireclay, heated either by a gas jet or by a small coke fire. It was found that this apparatus, crude as it was, answered the desired purpose, until some better arrangement was perfected, and the type was accordingly adopted throughout the whole system. It was quite recognized that this method still left much to be desired, and the economy resulting from the use of an improved form was very marked.

From a large number of trials very carefully carried out by Professor Gutermuth, it was found that more than 70 per cent. of the total number of calories in the fuel employed was absorbed by the air and transformed into useful work. Whether gas or coal be employed as the fuel, the amount required is so small as to be scarcely worth consideration; according to the experiments carried out, it does not exceed 0.09 kilo. per horse power and per hour, but it is scarcely to be expected that in regular practice this quantity is not largely exceeded. Professor Weyrauch has also carefully investigated this part of the subject and fully confirms, if he, indeed, does not go beyond Professor Gutermuth. He claims that the efficiency of fuel consumed in this way is six times greater than when burnt under a boiler to generate steam. He goes so far as to assert that with a good method of heating the air, not only can all the losses due to the production and the transmission of the compressed air be made good, but also that it will actually contain more useful energy at the motor than was expended at the central station in compressing it.