g edge of the screw enters the water without shock or
impact, as the advance of the leading edge per revolution will not be
greater than the advance of the vessel. When the pitch of a screw increases
in the direction of its diameter, the central part of the screw will
advance with only the same velocity as the water, so that it cannot
communicate any centrifugal velocity to the water; and the whole slip, as
well as the whole propelling pressure, will occur at the outer part of the
screw blades.

596. _Q._--Is there any advantage derived from these forms of screws?

_A._--There is a slight advantage, but it is so slight as hardly to balance
the increased trouble of manufacture, and, consequently, they are not
generally or widely adopted.

597. _Q._--What other kinds of screw are there proposing to themselves the
same or similar objects?

_A._--There is the corrugated screw, the arms of which are corrugated, so
as it were to gear with the water during its revolution, and thereby
prevent it from acquiring a centrifugal velocity. Then there is Griffith's
screw, which has a large ball at its centre, which, by the suction it
creates at its hinder part, in passing through the water, produces a
converging force, which partly counteracts the divergent action of the
arms. Finally, there is Holm's screw, which has now been applied to a good
number of vessels with success.

598. _Q._--Will you describe the configuration and action of Holm's screw?

_A._--First, then, the screw increases in the direction of its length, and
this increase is very rapid at the following edge, so that, in fact, the
following edge stands in the plane of the shaft, or in the vertical
longitudinal plane of the vessel. Then the ends of the arms are bent over
into a curved flange, the edge of which points astern, and the point where
this curved flange joins the following edge of the screw is formed, not
into an angle, but into a portion of a sphere, so that this corner
resembles the bowl of a spoon. When the screw is put into revolution, the
water is encountered by the leading edge of the screw without shock, as its
advance is only equal to the advance of the vessel, and before the screw
leaves the water it is projected directly astern. At the same time, the
curved flange at the rim of the screw prevents the dispersion of the water
in a radial direction, and it consequently assumes the form of a column or
cylinder of water, projected backward from the s