AB and BC, so far as the former (AB) is concerned, no alteration occurs
whether BK be straight or curved. But the other portion, BC, pressing
vertically against the concave surface, BK, becomes gradually diminished in
its velocity in relation to the earth, and produces and effect known as
"reaction." A combined operation of impact and reaction occurs by further
diminishing the distance which the bucket is allowed to travel, as, for
examples, to EF. Here the jet is impelled against the lower edge of the
bucket, B, and gives a pressure by its impact; then following the curve BK,
with a diminishing velocity, it is finally discharged at K, retaining only
sufficient movement to carry the water clear out of the machine. Thus far
we have considered the movement of jets and buckets along AB as straight
lines, but this can only occur, so far as buckets are concerned, when their
radius in infinite. In practice these latter movements are always curves of
more or less complicated form, which effect a considerable modification in
the forms of buckets, etc., but not in the general principles, and it is
the duty of the designer of any form of turbine to give this consideration
its due importance. Having thus cleared away any ambiguity from the terms
"impact," and "reaction," and shown how they can act independently or
together, we shall be able to follow the course and behavior of streams in
a turbine, and by treating their effects as arising from two separate
causes, we shall be able to regard the problem without that inevitable
confusion which arises when they are considered as acting conjointly.
Turbines, though driven by vast volumes of water, are in reality impelled
by countless isolated jets, or streams, all acting together, and a clear
understanding of the behavior of any one of these facilitates and concludes
a solution of the whole problem.

_Experimental researches_.--All experiments referred to in this paper were
made by jets of water under an actual vertical head of 45 ft., but as the
supply came through a considerable length of ½ in. bore lead piping, and