The consumption of air per brake horse power was reduced from 812 cubic feet per hour, a favorable duty in the single motor, to 720, and in the best result to 646 cubic feet with the two motors and double heaters. It should be added that these trials were carried out with steam engines but ill adapted for the purpose. It is to be regretted that the experiments of Professor Riedler could not have been conducted with more perfect appliances, but it must be borne in mind that the utilization of compressed air, especially as regards the motors, is still in a very imperfect stage, and that a great deal remains to be done before the maximum power available at the motor can be obtained. Investigations in this direction for a considerable time to come must be directed, therefore, toward improving the design and construction of the motors and the treatment of the air at the point of delivery into the engine. A large number of motors in use among the subscribers to the Compressed Air Company, of Paris, are rotary engines developing one horse power and less, and these in the early times of the industry were extravagant in their consumption, to a very high degree. To some extent this condition of things has been improved, chiefly by the addition of better regulating valves to control the air admission. As altered, the two horse power rotary motors, when employed as cold air engines, a method often desired in special industries, consume 1,059 cubic feet per hour and per indicated horse power; with a moderate degree of heating, say to 50 deg. Cent., this consumption falls to 847 cubic feet. The efficiency of this type of rotary motors with air heated to 50 deg. may now be assumed at 43 per cent., not a very economical result, it is true, and one that may be largely improved, yet it is evident that with such an efficiency the use of small motors in many industries becomes possible, while in cases where it is necessary to have a constant supply of cold air, economy ceases to be a matter of the first importance.