rtioned wheels; but it is
desirable to have the slip as small as is possible consistently with the
observance of other necessary conditions. The speed of the engine and also
the speed of the vessel being fixed, the diameter of the rolling circle
becomes at once ascertainable, and adding to this the slip, we have the
diameter of the wheel.

CONFIGURATION AND ACTION OF THE SCREW.

562. _Q._--Will you describe more in detail than you have yet done, the
configuration and mode of action of the screw propeller?

_A._--The ordinary form of screw propeller is represented in figs. 46 and
47; fig. 46 being a perspective view, and fig. 47 an end view, or view such
as is seen when looking upon the end of the shaft. The screw here
represented is one with two arms or blades. Some screws have three arms,
some four and some six; but the screw with two arms is the most usual, and
screws with more than three arms are not now much employed in this country.
The screw on being put into revolution by the engine, preserves a spiral
path in the water, in which it draws itself forward in the same way as a
screw nail does when turned round in a piece of wood, whereas the paddle
wheel more resembles the action of a cog wheel working in a rack.

[Illustration: Fig. 46. Fig. 47. ORDINARY FORM OF SCREW PROPELLER.]

563. _Q._--But the screw of a steam vessel has no resemblance to a screw
nail?

_A._--It has in fact a very close resemblance if you suppose only a very
short piece of the screw nail to be employed, and if you suppose, moreover,
the thread of the screw to be cut nearly into the centre to prevent the
wood from stripping. The original screw propellers were made with several
convolutions of screw, but it was found advantageous to shorten them, until
they are now only made one-sixth of a convolution in length.

564. _Q._--And the pitch you have already explained to be the distance in
the line of the shaft from one convolution to the next, supposing the screw
to consist of two or more convolutions?

_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 wo