ial there was a distance of 11/2 in. between the jet and point
of its contact with the plate, while in the second trial this space was
diminished to 1/2 in. It will be noticed that as this distance increases we
have augmented pressures, and these are not due, as might be supposed, to
increase of head, which is practically nothing, but they are due to the
recoil of a portion of the stream, which occurs increasingly as it becomes
more and more broken up. These alterations in pressure can only be
eliminated when care is taken to measure that only due to impact, without
at the same time adding the effect of an imperfect reaction. Any stream
that can run off at all points from a smooth surface gives the minimum of
pressure thereon, for then the least resistance is offered to the
destruction of the vertical element of its velocity, but this freedom
becomes lost when a stream is diverted into a confined channel. As pressure
is an indication and measure of lost velocity, we may then reasonably look
for greater pressure on the scale when a stream is confined after impact
than when it discharges freely in every direction. Experimentally this is
shown to be the case, for when the same oblong jet, discharged under the
same conditions, impinged vertically upon a smooth plate, and gave a
pressure of 71 units, gave 87 units when discharged into a confined
right-angled channel. This result emphasizes the necessity for confining
streams of water whenever it is desired to receive the greatest pressure by
arresting their velocity. Such streams will always endeavor to escape in
the directions of least resistance, and, therefore, in a turbine means
should be provided to prevent any lateral deviation of the streams while
passing through their buckets. So with screw propellers the great mass of
surrounding water may be regarded as acting like a channel with elastic
sides, which permits the area enlarging as the velocity of a current
passing diminishes. The experiments thus far described have been made with
jets of an oblong shape, and they give results differing in some degree
from those obtained with circular jets. Yet as the general conclusions from
both are found the same, it will avoid unnecessary prolixity by using the
data from experiments made with a circular jet of 0.05 square inch area,
discharging a stream at the rate of 40 ft. per second. This amounts to 52
lb. of water per minute with an available head of 25 ft., or 1,300
foot-pounds