A.—­Yes, that is what is meant by the pitch.  If a thread be wound upon a cylinder with an equal distance between the convolutions, it will trace a screw of a uniform pitch; and if the thread be wound upon the cylinder with an increasing distance between each convolution, it will trace a screw of an increasing pitch.  But two or more threads may be wound upon the cylinder at the same time, instead of a single thread.  If two threads be wound 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.