LOCOMOTIVE, COMPRESSED-AIR. 



513 



as to produce heat is called adiobatic compres- 

 sion ; while that which is slow enough to allow 

 the molecular particles to accommodate them- 

 selves to a more crowded condition, to shorter 

 paths and more frequent impact, without caus- 

 ing the disturbance, the temporary accelera- 

 tion of molecular velocity and augmented vio- 

 lence of impact, which is marked by a rise in 

 temperature, is denoted isothermal compres- 

 sion. If the energy stored in the elastic fluid 

 were instantly utilized, the work performed 

 would be the same, whether the pressure were 

 idiobatic or isothermal ; but if there is a suffi- 

 cient interval allowed to lapse for the energy 

 which has been converted into heat to be lost 

 by the radiation of the extra heat, there is a 

 waste to the extent of the power required to 

 produce the elevation of temperature. A cor- 

 responding loss of energy takes place in the 

 application of the stored-up power, which must 

 from the nature of the case be rapid also, and 

 a rapid expansion is attended by a diminution 

 of the potential energy equivalent to the loss 

 resulting from an equally rapid rate of com- 

 pression. This is due to the reduction in the 

 temperature produced by sudden expansion, 

 which lessens the elastic tension of the fluid in 

 a corresponding degree. This double loss of 

 efficient power attends every employment of 

 condensed air for the preservation for future 

 use or for the transmission of mechanical ener- 

 gy. It is used in mines for drilling, not because 

 it is an economical power, but because it can 

 be so handily applied and so readily regulated. 

 There are reasons which would make the use 

 of compressed-air locomotives desirable for 

 some purposes, though they should consume 

 considerably more fuel than steam locomotives. 

 French and Belgian engineers some time since 

 discovered a method of neutralizing the changes 

 in temperature due to compression and expan- 

 sion, and thus increasing the duty of pneu- 

 matic engines considerably. They kept the com- 

 pression and expansion isothermal by artificial 

 means, injecting a spray of cold water into the 

 compression -cylinder and a spray of hot water 

 or a jet of steam into the expansion -cylinder. 

 It was only a partial remedy, since the heat of 

 compression, which is absorbed by the parti- 

 cles of cold water, is not utilized, nor is the 

 heat that is imparted to the hot water which is 

 introduced into the expansion-cylinder to keep 

 up the temperature and effective force of 

 the compressed air while undergoing expan- 

 sion. Yet it served to preserve the kinetic 

 equilibrium of the gas, and enhanced very 

 materially the efficiency of the air-engine. 

 The loss of power attending this artificial 

 method of equalizing the temperature, and 

 making the tension of the confined air vary 

 uniformly with the pressure applied, is not very 

 great; and, what is of the highest importance 

 as regards the employment of compressed air 

 in locomotives, it is proved by theory and sub- 

 stantiated in practice that this loss is directly 

 proportional to the degree of compression. 

 VOL. xxi. 33 A 



The size and weight of the reservoirs and oth- 

 er apparatus consequently depend in a great 

 measure upon the degree of compression which 

 can be practicably attained. An improvement 

 upon the Continental method of maintaining a 

 steady temperature in the air under pressure 

 has been in use for some time in the pneu- 

 matic apparatus of the Royal Arsenal at Wool- 

 wich, England, used for compressing air to a 

 high degree of pressure for the propulsion of 

 torpedoes. The injection of the spray into the 

 cylinders requires the use of valves and other 

 attachments. More or less leakage will occur 

 through the joints. If the compressed air is 

 used at a high degree of pressure, this would 

 prove a fatal defect. To obviate this leakage, 

 the Woolwich engineers employed in place of 

 the injecting apparatus a simple jacket of cold 

 water around the compressing cylinder. This 

 technical improvement, allowing of the use of 

 compressed-air power at very high pressures, 

 led Colonel Beaumont, of the Royal Arsenal, to 

 the study of a system of compressed-air loco- 

 motives worked under extreme pressure, in vir- 

 tue of the principle that high degrees of com- 

 pression, when kept isothermal, are obtainable 

 at virtually no greater relative cost than lower 

 pressures. He obviated the loss of power in 

 expansion by steam-jacketing the expansion 

 cylinders, which improvement afforded all that 

 was wanting for the isothermal compression 

 and expansion of air at high pressures. Colonel 

 Beaumont has employed himself a long time 

 upon the elaboration of his system of com- 

 pressed-air motors, and has developed an air- 

 locomotive which works satisfactorily as far 

 as experimental tests can show, and in long 

 and searching trials has not only proved itself 

 practicable, but in the points of reliability, 

 durability, manageability, noiselessness, and, 

 above all, of economy, it appears to be a com- 

 plete success, which promises to remove the 

 question of a compressed-air locomotive-tn- 

 gine finally out of the category of mechanical 

 problems. 



Colonel Beaumont has built an engine which 

 performs the trip from the arsenal to Dartford 

 and back, a total distance of about thirty miles, 

 with one charge of compressed air, taking sixty- 

 three minutes. His locomotive weighs ten 

 tons. It can carry a load of sixteen tons up a 

 moderate grade. The reservoir is charged with 

 one hundred cubic feet of air under a pressure 

 of one thousand pounds on the square inch. 

 The first locomotive made on his system had 

 three cylinders on each side for the purpose of 

 expanding the air down. It is parted with in 

 the last at little above atmospheric pressure. 

 He uses no expanding apparatus to reduce the 

 pressure before turning on the air, but lets it 

 into the cylinders at the full pressure of one 

 thousand pounds. The cylinders are completely 

 air-tight practically, being able to retain the 

 charge for several hours without any material 

 reduction of pressure. The form first used 

 consisted of a number of strong tubes con- 



