In using a pneumatic motor there are three different machines all receiving their energy from the boiler steam. 1. The engine which drives the compres.sing machine; 2. The compressing machine itself; and 3. The engine which drives the locomotive. It is estitnated that the loss of power in all these amounts to about one-half of that con tained in the steam boiler of the pumpino- engine. One of the earliest compressed air locomotives was devised by M. Ribourt,°the engineer at St. Gothard (see TUNNEL), for hauling debris from the tunnel. 31. Ribourt's method for equalizing the pressure upon the driving pistons was the employment of a sliding cylinder inside of and concentric with the cylinder in which the drivino• piston moves. This inside cylinder is controlled by a spiral spring which is connected with the piston rod. Compressed air at the initial pressure enters the cylinder between the piston heads. Within this space it therefore has no effect, but it passes front this chamber through orifices into an outer jacket, and thence again on the further side of OLIC of the piston heads, that one opposite the end to which the spring is applied. These orifices pass through both inside and out side cylinders, and their capacity depends upon the relative positions of the two cylin ders. The adjustments of the different parts of the apparatus are so made that, when the air passes through the jacket to the outer surface of the piston head upon which it acts at its initial pressure the grifices in the cylinders do not exactly coincide, and their capacity is therefore diminished. As, however, the tension of the air dirOnishes, the spiral spring, acting against the pneumatic pressure, forces the inside cylinder farther back, at the sante brae increasing the capacity of the openings in the two cylinders by making them more nearly coincide. This increase of capacity of orifice is in the inverse ratio to the pressure, and the action is reciprocal and continuous. Considerable modifi cations have been made in motors running upon tramways in Glasgow, Paris, and New York. M. Alekarski has successfully propelled motors in France with compressed air at 450 lbs. per sq.in., or 30 atmospheres. The ordibary high-pressure locomotive engine is the form used, but the compressed air before reaching the cylinders is forced through a tank of hot water at about 220° F., by which means it becomes saturated with steam. An equalizing throttle-valve is placed on the top of the hot-water reservoir, for the purpose of regulating the pressure upon the pistons. Two of the locomotives were exhibited at the Paris exposition of 1878, one a car motor, the other a separate motor. The latter could draw a car containing 30 passengers from 10 to 11 m. on a level, and
could ascend a grade of '5 to 100. Further improvements, it is said, have been intro duced on motors which have been running on the Second avenue railroad in New York. One of the improvements is the passing of the compressed air through water heated to about 328'. It is claimed that the motors have worked successfully, and at a less cost than when horses are used for the same amount of work. Sonte engineers, however, do not accept these estimates, and it is declared that the experience at Glasgow, where both compressed air and steam motors have been used, indicates that the pneumatic motor requires more than four times the expenditure of steam to perform the same work that the steam motors do: and a leading French engineer says that at Paris it is estimated that the cost of motive power on street railways, calling horse-power 100, will be, for ,compressed air, 64, and for steam power, 20, making compressed air a little more than three times as expensive as steam. It must, however, be understood that but a short time has elapsed since the first trials were made, and yet that considerable progress has been made—perhaps oTeater than has ever attended the development of any similar invention. A pamphlet issued by the pneumatic tramway engine company of New York a letter from gen. Herman Haupt, its consulting engineer, in which he says ." that although one-half the power of the stationary engine is lost in compressing air, yet the econorny of fuel can be made so great that a given amount of power in compressed air is secured at one-half the cost of the direct application of steam to motors. The difference in specific heat of water and of air also is important as regards the advantage in economy of air. See HEAT, ante; Specific Heat. Geu. Haupt again says: " By a simple device of heating the air by passing it through a, tank of water it is claimed as the result of constant practice in Paris, confirmed by recent experiments on the Second avenue railroad, that the capacity for work is doubled, or the gain 100 per cent, making the economy of power, as compared with the direct application of steam to street motors, measured fla it should be by the coal consumed, four to one in favor of compressed air." Again: " The motor cylinders are so arranged that in descending steep grades they act as • pumps, and at the same time as brakes, by which nieans it is found, as stated by the company's engineer, Mr. Hardie, that in running down grade on the Second avenue. railroad, pumping back against a pressure of 200 lbs. in the receiver, the pressure was increased 7 lbs. in a distance of four-tenths of a mile."