Duty of Aiir-Motors.-In many English mines experiments have been conducted with a view to determining the fraction of absolute work theoretically transmitted by air delivered, which machines, driven by said air, return in the form of effective work. This work has always represented 55 to 75 per cent. of the absolute work, which corresponds to the consumption of compressed air. At St. Gothard Tunnel, M. Rebourt, by direct experiment upon compressed-air locomotives, determined that the relation of tractile work to the theoretic work of air expended was comprised between .50 and .60. If, instead of seeking a ratio between the effective work and the theoretic work contained in the air expended, we determine the same between the first and the work expended to compress the air so as to obtain the total useful effect of the entire system, or, in other words, for the fraction of work expended by primary motor which is returned from the shaft of the compressed -air engine, the relation is found to be between 20 and 25 per cent. at high pressure, or 35 and 40 per cent. at low pressure.