452 REPORT — 1889. 



economical ratio of expansion for the not very high available initial 

 pressure. I believe that in every case heaters are provided for these engines, 

 although in some instances, where both power and refrigeration are re- 

 quired, they are used sparingly, or not at all, in order to take advantage 

 of the cooling due to expansion. 



After this short description of the general arrangements of the Paris 

 company's work, I come now to the experiments which I made to ascertain 

 its efficiency. Starting from the main engines at the central station, the 

 particular matter which I had to determine was the indicated horse-power 

 which would be shown by a small motor three or four mUes from St. 

 Fargeau for each indicated horse-power expended by the main engines on 

 the air which passed through that motor. The ratio thus obtained would 

 be the total indicated efficiency of the whole system of transmission. This 

 ratio is in reality the product of a number of separate efficiencies, and the 

 separate determination of these formed a necessary check on the value of 

 the total efficiency. These separate efficiencies may be summarised as 

 follows : — 



1. Mechanical efficiency of main engines, or ratio of work done in 

 compressors to indicated work in steam-cylinders. 



2. Efficiency of compressors, or ratio of maximum work which could 

 be done in a motor by each cubic foot of compressed air at 70° Fahr., to 

 the work actually done in compressing that air. 



3. Efficiency of mains, or ratio in which the capacity of the compressed 

 air for doing work is reduced by friction and leakage. 



4. Efficiency of reducing-valves, or ratio in which the capacity of the 

 compressed air for doing work is reduced by the lowering of its initial 

 pressure at the motor. 



5. Indicated efficiency of motor, or ratio in which the actual indicated 

 work done falls short of the maximuna work which the quantity of air 

 measured through the meter could do after passing the reducing-valve. 



The product of these five efficiencies is the total efficiency of transmis- 

 sion without the use of a heater. When a heater is used the matter is 

 somewhat more complicated. All the ratios given above represent what 

 may be called mechanical efficiencies, all of them have unity for their maxi- 

 mum attainable value. It is therefore not possible to introduce in direct 

 combination with them a thermodynamic efficiency (ratio of additional heat 

 supplied to additional work done), which has for its maximum value not 

 unity but 0'3 or some similar small value. This could only be done if 

 the measurement of efficiency had started originally from the heat given 

 to the steam instead of from the indicated horse-power, and this 

 would have given numbers having a minimum of practical value or con- 

 venience. Probably the best practical measure of the efficiency of the 

 whole transmission, when using heated air, is obtained by finding the 

 equivalent in indicated horse-power at the central station of the coke used 

 in the heater, and adding this to the indicated horse-power actually used. 

 It would not be possible by the expenditure of this or any other amount 

 of indicated horse-power at the central station to obtain the same results 

 as by heating the air just before entering the motor ; but that, of course, 

 does not affect the question before us. 



The determination of the indicated horse-power of the main engines 

 presented no difficulty. I measured it on one pair of engines at different 

 speeds from 21 to 44 revolutions per minute. At 31 '5 revolutions per 

 minute it amounted to 254-9, and at all speeds it was approximately B'l 



