From these experiments it would appear that, assuming a speed of 21 ft. per second, a loss in pressure of one atmosphere would correspond to a distance of 20 kilometers; that is to say, a central station could extend its mains on all sides with a radius of 20 kilometers, and the motors at the ends of the lines would receive the air at a pressure 15 lb. less than at the central station. Professor Riedler states that as an actually measured result, the velocity of the air through the mains of the St. Fargeau system is 19 ft. 8 in. per second, and that the loss in pressure per kilometer is 0.07 atmosphere. From this it follows that including the resistances due to the four reservoirs, and other obstructions actually existing, an allowance of one atmosphere loss on a 14 kilometer radius is ample. By increasing the initial pressure of the air, much better results can be obtained, and future attention in practice should be devoted to this point. The amount of work required to compress air does not increase in the same ratio as the pressure, and for this reason considerable economy can be effected at the first stage, and the loss in the mains will be reduced. Passing to another point of the same subject, Professor Riedler considers the best dimensions that should be given to the mains. Resistance decreases with an increase in the diameter of these and in direct ratio to their diameter; for this reason--still assuming a pressure corresponding to a velocity of 20 ft. per second--with a fall of one atmosphere, a length of 40 kilometers could be succesfully worked. The mains of the new _réseau_ for the Quai de la Gare station are 19.69 in. in diameter; they are built up of steel plates riveted, and this Professor Riedler considers to have been a serious error on account of the extra resistance offered by the large number of rivet heads. The following may be taken as a brief summary of Professor Riedler's conclusions: Recent improvements in central station practice have resulted in an increased efficiency of about 30 per cent. in the compressors, but this benefit can only be realized when the new station is in operation. That the small and very imperfect air engines in use on the system give an efficiency of 50 per cent., while with ordinary steam engines driven by air an efficiency of 80 per cent. can be reached with a very small expenditure of fuel for heating the air before admitting it into the motor. That special attention should be given to the improvement of air engines, and that with increased initial pressures at the central station the distance of the transmission can be very considerably augmented. Finally, Professor Riedler claims that power can be transmitted by compressed air more conveniently and more economically than by any other means. * * * * * [Continued from SUPPLEMENT, No. 802, page 12810.] THE BUILDERS OF THE STEAM ENGINE--THE FOUNDERS OF MODERN INDUSTRIES AND NATIONS.[1]