ON THE TRANSMISSION OF POWER BY COMPRESSED AIR. 457 



the same motor, working as nearly as possible at the same power and 

 speed, and with exactly the same pressure, but passing the air between 

 the meter and the engine through such a heating stove as I have already 

 described. I weighed all the coke used, and read the temperatures every 

 five minutes during a four hours' trial. The air was heated in passing 

 through the stove up to 315° Fahr., with a consumption of about 039 lb. 

 coke per indicated horse-power per hour. As the admission temperature 

 on the cold trials was 83° Fahr. ' only, this corresponds to an increase of 

 about 42 per cent, in the volume of the air, and should therefore (had the 

 indicated e6Bciency remained the same) have been accompanied by a 

 decrease of air consumption in the ratio —, or 070. The air actually 



used was 665 cubic feet per indicated horse-power per hour, which is 

 0'75 of the 890 cubic feet formerly required, so that the full economy is 

 very nearly realised. The indicator diagrams are shown in Fig. 5, and 

 are represented by K P Q O in Fig. 3. An air consumption of 665 cubic 

 feet per indicated horse-power per hour corresponds to an indicated 

 efficiency over the whole system of 0"52 ; in other words, 1-92 indicated 

 horse-power is required at St. Fargeau per indicated horse-power at the 

 motor. The mechanical efficiency of the motor was very mnch greater 

 hot than cold, rising to 081. Hence about 2^ indicated horse-power at 

 St. Fargeau gave one brake horse-power at the motor. These figures, 

 however, take no account of the coke burnt in the heater, and are, there- 

 fore, only to be considered as apparent efficiencies. Allowing for the 

 value of the coke in the manner already described, the real indicated 

 efficiency of the whole transmission is 0"47. 



A shorter expeinment with slightly higher temperatures and con- 

 siderably larger indicated horse-power gave still more economical results, 

 the air consumption falling to 623 cubic feet per indicated horse-power 

 per hour, an "apparent" indicated efficiency of 056. This experiment 

 was not, however, of sufficient duration to allow of coke measurement. 



As to the value of the preliminary heating, the figures which I have 

 given show that it caused a saving of 225 cubic feet of air per indicated 

 horse-power per hour, at a cost to the consumer of about 0'4 lb. of coke 

 per indicated horse-power per hour. I do not doubt that the stoking of 

 the heater during my experiment was much more careful than it would 

 be in ordinary practice, although, on the other hand, it would not be 

 difficult to design a more economical stove. If, however, the coke 

 consumption were even doubled, it would only amount lo 72 lbs. per day of 

 nine hours for 10 indicated horse-power, the value of which might be 6rf. 

 or 7d. The air saved under the same circumstances would be over 20,000^ 

 cubic feet, the cost of which, at the high rate charged in Paris, would be 

 7s. 3d. There is no doubt therefore that to obtain the maximum of 

 economy the preliminary heating of the air should be carried as far as is 

 practicable. 



Of course heating the air serves the purpose also of preventing any 

 chance of the exhaust pipe becoming ice-clogged. I found this to happen 

 once or twice when working with cold air, its occurrence depending 

 rather on the amount of moisture in the air than on the exhaust tempera- 

 ture, for the engine, after running freely with an exhaust of —35° Fahr., 



' This somewhat high admission temperature was the only point in which the 

 motor at St. Fargeau differed from those in Paris, where I found the admission tem- 

 perature to be from 69° to 71° Fahr. 



