figures, and they justify the conclusion that the inclinations of blades
found out by practice ought to be arrived at, or at any rate approached, by
any sound and reliable theory; and that blades of whatever form must not
transgress far from this inclination if they are to develop any
considerable efficiency. Indeed, many favorable 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