expended at the central station in compressing it.

According to Professor Riedler, from 15 to 20 per cent. above the
power at the central station can be obtained by means at the disposal
of the power users, and it has been shown by experiment that by
heating the air to 250 deg. Cent. an increased efficiency of 30 per
cent. can be obtained. Better results than those heretofore obtained
may, therefore, be confidently expected with a more perfect and
economical application of the fuel in heating the air, and a better
means of regulation in admitting it to the motors. In his report
Professor Riedler indicates a method by the use of which he considers
considerable advantages may be secured. This is the heating the air in
two stages instead of at one operation, and passing it through two
motors, to the first of which the air is admitted heated only to a
moderate extent; the exhaust from this motor then passes into a second
heater and thence into the second motor. A series of experiments with
this arrangement were recently carried out.

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