ervention of toothed wheels,--the engines themselves moving with the
usual velocity of paddle engines; whereas in direct acting engines the
required velocity of rotation is obtained by accelerating the speed of the
engines, and which are connected immediately to the screw shaft.

122. _Q._--Will you describe some of the principal varieties of geared
engines?

_A._--A good many of the geared engines for screw vessels are made in the
same manner as land engines, with a beam overhead, which by means of a
connecting rod extending downwards, gives motion to the crank shaft, on
which are set the cog wheels which give motion to pinions on the screw
shaft,--the teeth of the wheels being generally of wood and the teeth of
the pinions of iron. There are usually several wheels on the crank shaft
and several pinions on the screw shaft; but the teeth of each do not run in
the same line, but are set a little in advance of one another, so as to
divide the thickness of the tooth into as many parts as there are
independent wheels or pinions. By this arrangement the wheels work more
smoothly than they would otherwise do.

123. _Q._--What other forms are there of geared screw engines?

_A._--In some cases the cylinders lie on their sides in the manner of the
cylinders of a locomotive engine. In other cases vertical trunk engines are
employed; and in other cases vertical oscillating engines.

124. _Q._--Will you give an example of a geared vertical oscillating
engine?

_A._--The engines of a geared oscillating engine are similar to the paddle
wheel engines (figs. 27 and 28), but the engines are placed lengthways of
the ship, and instead of a paddle wheel on the main shaft, there is a
geared wheel which connects with a pinion on the screw shaft. The engines
of the Great Britain are made off the same patterns as the paddle engines
constructed by Messrs. John Penn & Son, for H.M.S. Sphinx. The diameter of
each cylinder is 82-1/2 inches, the length of travel or stroke of the
piston is 6 feet, and the nominal power is 500 horses. The Great Britain is
of 3,500 tons burden, and her displacement at 16 feet draught of water is
2,970 tons. The diameter of the screw is 15-1/2 feet, length of screw in
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 stai