und upon
it they will trace a double-threaded screw; if three threads be wound upon
it they will trace a treble-threaded screw; and so of any other number. Now
if the thread be supposed to be raised up into a very deep and thin spiral
feather, and the cylinder be supposed to become very small, like the newel
of a spiral stair, then a screw will be obtained of the kind proper for
propelling vessels, except that only a very short piece of such screw must
be employed. Whatever be the number of threads wound upon a cylinder, if
the cylinder be cut across all the threads will be cut. A slice cut out of
the cylinder will therefore contain a piece of each thread. But the
threads, in the case of a screw propeller, answer to the arms, so that in
every screw propeller the number of threads entering into the composition
of the screw will be the same as the number of arms. An ordinary screw with
two blades is a short piece of a screw of two threads.

565. _Q._--In what part of the ship is the screw usually placed?

[Illustration: Fig. 48]

_A._--In that part of the run of the ship called the dead wood, which is a
thin and unused part of the vessel just in advance of the rudder. The usual
arrangement is shown in fig. 48, which represents the application to a
vessel of a species of screw which has the arms bent backwards, to
counteract the centrifugal motion given to the water when there is a
considerable amount of slip.

566. _Q._--How is the slip in a screw vessel determined?

_A._--By comparing the actual speed of the vessel with the speed due to the
pitch and number of revolutions of the screw, or, what is the same thing,
the speed which the vessel would attain if the screw worked in a solid nut.
The difference between the actual speed and this hypothetical speed, is the
slip.

567. _Q._--In well formed screw propellers what is the amount of slip found
to be?

_A._--If the screw be properly proportioned to the resistance that the
vessel has to overcome, the slip will not be more than 10 per cent., but in
some cases it amounts to 30 per cent., or even more than this. In other
cases, however, the slip is nothing at all, and even less than nothing; or,
in other words the vessel passes through the water with a greater velocity
than if the screw were working in a solid nut.

568. _Q._--Then it must be by the aid of the wind or some other extraneous
force?

_A._--No; by the action of the screw alone.

569. _Q._--But how is