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.