mount of pressure on the pistons, and the velocity
of their motion relatively with the velocity of advance made by the screw,
supposing it to work in a solid nut, it is easy to tell what the thrust of
the screw would be if it were cleared of the effects of friction and other
irregular sources of disturbance. The thrust, in fact, would be at once
found by the principle of virtual velocities; and if we take this
theoretical thrust and diminish it by one fourth to compensate for friction
and lateral slip, we shall have a near approximation to the amount of
thrust that will be actually exerted.[1]

[1] See Treatise on the Screw Propeller, by J. Bourne, C. E.

COMPARATIVE ADVANTAGES OF DIFFERENT SCREWS.

592. _Q._--What species of screw do you consider the best?

_A._--In cases in which a large diameter of screw can be employed, the
ordinary screw or helix with two blades seems to be as effective as any
other, and it is the most easily constructed. If, however, the screw is
restricted in diameter, or if the vessel is required to tow, or will have
to encounter habitually strong head winds, it will be preferable to employ
a screw with an increasing pitch, and also of such other configuration that
it will recover from the water some portion of the power that has been
expended in slip.

593. _Q._--How can this be done?

_A._--There are screws which are intended to accomplish, this object
already in actual use. When there is much slip a centrifugal velocity is
given to the water, and the screw, indeed, if the engine be set on when the
vessel is at rest, acts very much as a centrifugal fan would do if placed
in the same situation. The water projected outward by the centrifugal force
escapes in the line of least resistance, which is to the surface; and if
there be a high column of water over the screw, or, in other words, if the
screw is deeply immersed, then the centrifugal action is resisted to a
greater extent, and there will be less slip produced. The easiest
expedient, therefore, for obviating loss by slip is to sink the screw
deeply in the water; but as there are obvious limits to the application of
this remedy, the next best device is to recover and render available for
propulsion some part of the power which has been expended in giving motion
to the water. One device for doing this consists in placing the screw well
forward in the dead wood, so that it shall be overhung by the stern of the
ship. The water force