rable results obtained by
propellers are not due to their peculiarities, but only to the fact that
they have been made with an inclination of blade not far from 42 deg. to
the plan of rotation. Referring to column 4, and accepting the case of
water flowing through a smooth tube as analogous to that of a current
flowing within a large body of water, it appears that the inclination
necessary to give the highest resultant pressure is an angle of 49 deg.,
and this corresponds closely enough with the angle which practical
constructors of screw propellers have found to give the best results.
Until, therefore, we can deal with currents after they have been
discharged from the blades of a propeller, it seems unlikely that anything
can be done by alterations in the pitch of a propeller. So far as concerns
theory, the older turbines were restricted to such imperfect results of
impact and reaction as might be obtained by turning a stream at right
angles to its original course; and the more scientific of modern turbine
constructors may fairly claim credit for an innovation by which practice
gave better results than theory seemed to warrant; and the consideration of
this aspect of the question will form the concluding subject of the present
paper. Referring again to Fig. 3, when a current passes round such a curve
as the quadrant of a circle, its horizontal reaction appears as a pressure
along _c_ B, which is the result of the natural integration of all the
horizontal components of pressures, all of which act perpendicularly to
each element of the concave surface along which the current flows. If, now,
we add another quadrant of a circle to the curve, and so turn the stream
through two right angles, or 180 deg., as shown by Fig. 4, then such a
complete reversal of the original direction represents the carrying of it
back again to the highest point; it means the entire destruction of its
velocity, and it gives the maximum pressure obtainable from a jet of water
impinging upon a surface of any form whatsoever. The reaction noticed in
Fig. 3 as acting along _c_ B is now confronted by an impact of the now
horizontal stream as it is turned round the second 90 deg. of curvature,
and reacts also vertically downward. It would almost seem as if the first
reaction from B to F should be exactly neutralized by the second impact
from F to D. But such is not the case, as experiment shows an excess of the
second impact over the first reaction amounti