made there are forty-five turbines through which the steam passes in
succession, expanding in each, until it is finally exhausted.

[Illustration: THE COMPOUND STEAM TURBINE.]

The theoretical efficiency of a motor of this kind is arrived at by Mr.
Parsons in the following manner:

The efflux of steam flowing from a vessel at 15.6 lb. per square inch
absolute pressure through an orifice into another vessel at 15 lb.
pressure absolute is 366 ft. per second, the drop of pressure of 0.6 lb.
corresponding to a diminution of volume of 4 per cent. in the opposite
direction. The whole 45 turbines are so proportioned that each one,
starting from the steam inlet, has 4 per cent. more blade area or
capacity than that preceding it. Taking the pressure at the exhaust end
to be 15 lb. absolute, that at the inlet end will be 69 lb. above the
atmosphere. The steam enters from the steam pipe at 69 lb. pressure, and
in passing through the first turbine it falls 2.65 lb. in pressure, its
velocity due to the fall being 386 ft. per second, and its increase of
volume 3.85 per cent. of its original volume. It then passes through the
second turbine, losing 2.55 lb. in pressure, and gaining 3.85 per cent.
in volume, and so on until it reaches the last turbine, when its
pressure is 15.6 lb. before entering, and 15 lb. on leaving. The
velocity due to the last drop is 366 ft. per second. The velocity of the
wheels at 9,200 revolutions per minute is 150 ft. per second, or 39.9
per cent. of the mean velocity due to the head throughout the turbines.
Comparing this velocity with the results of a series of experiments made
by Mr. James B. Francis on a Tremont turbine at Lowell, Mass., it
appears that there should be an efficiency of 72 per cent. if the
blades be equally well shaped in the steam as in the water turbine, and