the line of the shaft, 3 feet 2 inches, and the pitch of the screw, 19
feet.

125. _Q._--What do you mean by the pitch of the screw?

_A._--A screw propeller may be supposed to be a short piece cut off a screw
of large diameter like a spiral stair, and the pitch of a spiral stair is
the vertical height from any given step to the step immediately overhead.

126. _Q._--What is the usual number of arms?

_A._--Generally a screw has two arms, but sometimes it has three or more.
The Great Britain had three arms or twisted blades resembling the vanes of
a windmill. The multiple of the gearing in the Great Britain is 3 to 1, and
there are 17-1/2 square feet of heating surface in the boiler for each
nominal horse power. The crank shaft being put into motion by the engine,
carries round with it the great cog wheel, or aggregation of cog wheels,
affixed to its extremity; and these wheels acting on suitable pinions on
the screw shaft, cause the screw to make three revolutions for every
revolution made by the engine.

127. _Q._--What are the principal varieties of direct acting screw engines?

_A._--In some cases four engines have been employed instead of two, and the
cylinders have been laid on their sides on each side of the screw shaft.
This multiplication of engines, however, introduces needless complication,
and is now but little used. In other cases two inverted cylinders are set
above the screw shaft on appropriate framing; and connecting rods attached
to the ends of the piston rods turn round cranks in the screw shaft.