hree pairs of rollers, as shown, so that its axis
of rotation is the same as that of the shaft, S; and it is toothed
externally as well as internally. The strands pass through the hollow
axes of the pinions, and thence each to its own opening through the
laying-top, T, fixed upon S, which completes the operation of twisting
them into a rope. The annular wheel, A, it will be perceived, may be
driven by a pinion, E, engaging with its external teeth, at a rate of
speed different from that of the central shaft; and by varying the speed
of that pinion, the velocity of the wheel, A, may be changed without
affecting the velocity of S.

It is true that in making a certain kind of rope, the velocity ratio of A
and S must remain constant, in order that the strands may be equally
twisted throughout; but if for another kind of rope a different degree of
twist is wanted, the velocity of the pinion, E, may be altered by means of
change-wheels, and thus the same machine may be used for manufacturing
many different sorts.

The second combination of this kind was devised by the writer as a
"tell-tale" for showing whether the engines driving a pair of twin
screw-propellers were going at the same rate. In Fig. 33, an index, P, is
carried by the wheel, F: the wheel, A, is loose upon the shaft of the
train-arm, which latter is driven by the wheel, E. The wheels, F and _f_,
are of the same size, but _a_ is twice as large as A; if then A be driven
by one engine, and E by the other, at the same rate but in the opposite
direction, the index will remain stationary, whatever the absolute
velocities. But if either engine go faster than the other, the index will
turn to the right or the left accordingly. The same object may also be
accomplished as shown in Fig. 34, the index being carried by the
train-arm. It makes no difference what the actual value of the ratio A/_a_
may be, but it must be equal to F/_f_: under which condition it is evident
that if A and F be driven contrary ways at equal speeds, small or great,
the train-arm will remain at rest; but any inequality will cause the index
to turn.

In some cases, particularly when annular wheels are used, the train-arm
may become very short, so that it may be impossible to mount the
planet-wheel in the manner thus far represented, upon a pin carried by a
crank. This difficulty may be surmounted as shown in Fig. 35, which
illustrates an arrangement originally forming a part of Nelson's steam
steer