660 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



stage of 40 miles. Air being denser than steam in the propor- 

 tion of 1^ to 8, and, so far as your committee are informed, a 

 cubic foot of steam being equal to a cubic foot of air, it would 

 require 10| tons of air for 40 miles. The vessel to hold this 

 air, even if welded, must be eight times heavier than the air ; 

 hence, 95 tons will be the weight of the charged reservoir for 40 

 miles; and a cylinder six feet in diameter and 100 feet long would 

 be required for it at the pressure of 1,000 lbs. per inch. This 

 excessive bulk is impracticable ; a quarter or third of it is as 

 much as could be allowed in practice ; and at least two stops 

 would be required in 40 miles. 



To stop and start such a train involves a loss of $1.20 and 

 $2.40 for two stops, or six cents per mile, which is three-quarters 

 of the cost of coke on such trains. Besides, the time of passen- 

 gers is of much greater value than the whole motive power ; if 

 the two stops should waste eight minutes, it would waste eight 

 dollars in the time of passengers, or 20 cents per mile, which 

 would drive the best class of passengers to the steam railways. 



These considerations are sufficient to account for the disap- 

 pointment of those projectors, even if there be no other defect 

 in their system, and all they claim — a saving of one-half — be un- 

 questionable. Mr. Carson, aware of these objections, devised a 

 means of charging the air holders while the train is running, so 

 that the air locomotive need not be heavier than a steam loco- 

 motive without its tender. It was this idea which induced him 

 to engage in this means of propulsion in England. But on seeing 

 our street railways, which are so short that there is no occasion 

 to re-charge between the termini, he thought it advisable to in- 

 troduce the air systein first on these lines ; and for this purpose 

 he has designed a car whose frame is composed of six inch lap- 

 welded tubes, in which the air is held. 



If we reduce the distance to a tenth, we may reduce the weight 

 to a tenth, or 9^ tons, for a 100 ton train. The boiler and water 

 weigh 16 tons. Hence, we find an advantage in favor of air on 

 short lines. 



As to the cost of compression, it is less at a low density than 

 at a high density, and there is room in a car for air at 300 or 

 400 lbs. But double this density is necessary for 10 miles ; and 

 at most high pressures, four times the power it can give out 

 when worked Avithout expansion, is required to compress it. But 

 it is practicable, with the link, to work it expansively, so as to 



