LOCOMOTIVE, COMPRESSED Ain. The attention of engineers has for a few years past been directed to the construction of locomotives using compressed air instead of steatn. Compressed air for driving stationary engines for rock-th in tunnels has been in use for some time, but in these the cotnpressed air was (Meetly furnished by a pump driVen either by steam or water-power, the latter being preferred where conve nient. At the present time the application of compressed air to locomotives is thought practical only for short lines and where steam is objectionable, but it is possible that in the future long lines of railway may be furnished with pneumatic pipes, or with pump ing-stations, and receive their naotor power in tbis manner. Compressed air street-motors have been used in Glasgow, Paris, and New York; and two Scotti'sh engineers, Robert Hardie and John James, have been and still are engaged in this country upon the prob lem. It is said that the pneumatic enfines devised by them, which have been running at intervals on the Harlem portion of the Second avenue surface road, between 96th and 130th streets, have proved so satisfactory that no doubts are entertained by the pneu matic tramway company that before many years this mode of propelling passenger cars on comparatively short distances will be generally adopted. It is believed that the prop ertics of atmospheric air have not been utilized to anythiug near their natural limits.
The first problem in compressed-air locomotion is to compress and store air in tu' resevoir of suitable dimensions to be carried OD a street-motor or car. In order that such car may be driven several miles and make numerous stops, a considerable amount of energy must be stored at the commencement of the trip, unless pneumatic pipe's be laid along the line. In any case a certain distance has to be run before the compressed air reservoir can be replenished. The reservoir of compressed air may, therefore, be com pared to the fuel of a steam-engine, although the air derives its energy from the fuel which supplies the compressing steam-engiue. This comparison may show the impor
tance of furnishinfr the motor with a conveniently disposed air-chamber filled with highly compressed air, afd also of maintaining an equable pressure upon the driving pistons, while the cornpressed air is constantly diminishing in tension by its escape in performing its work. It is said by engineers who have given practical attention to the subject that it will be desirable to use an initial pressure of about 500 lbs. to the sq.in., which is the equivalent of about 33 atmospheres. A pressure of 300 lbs. to the sq.in., or 24 atmospheres, has been found practicable, and most motors have hitherto been run with this pressure. Of course, the compression of the air converts a vast amount of latent into sensible heat. See HEAT, ante; Latent Heat. This energy is lost because there is no way to prevent the sensible heat from being conducted away or dispersed. If the air be introduced into the motor reservoirs in the heated and dry. condition which it attains in the pump cylinder, it would not be fit to perform its duty In the driving cylinders of the motor. It would not, however, retain its expanded volume in the motor reservoir without being kept heated. Before entering these reservoirs it must be cooled, and it is not improbable that the heat with which it parts on cooling may be utilized in produc ing a part of the steam for the pumping engine. The methods of cooling are various; those employed in compressing-engines for furnishing air directly to stationary air engines have the pump cylinder surrounded by a cold-water jacket, or have a circula tion of cold water in the cylinder head, or have sprays of cold water forced into the pump cylinder. The air for a. store cylinder from Ni Inch motors take their compressed, air may, however, be more conveniently cooled by passing it through a tank of cold water.