was built, can examine this engine without having an
increased respect for the talents of Richard Trevithick, a man to whom we
owe so much and whose labors have as yet met with such scant
recognition.--_Engineering._

* * * * *

[Continued from SCIENTIFIC AMERICAN SUPPLEMENT, No. 451, page 7192.]

PLANETARY WHEEL TRAINS.

By Prof. C.W. MacCORD, Sc. D.

IV.

The arrangement of planetary wheels which has been applied in practice to
the greatest extent and to the most purposes, is probably that in which
the axial motions of the train are derived from a fixed sun wheel.
Numerous examples of such trains are met with in the differential gearing
of hoisting machines, in portable horse-powers, etc. The action of these
mechanisms has already been fully discussed; it may be remarked in
addition that unless the speed be very moderate, it is found advantageous
to balance the weights and divide the pressures by extending the train arm
and placing the planet-wheels in equal pairs diametrically opposite each
other, as, for instance, in Bogardus' horse power, Fig. 31.

[Illustration: PLANETARY WHEEL TRAINS.]

In trains of this description, the velocity ratio is invariable; which for
the above-mentioned objects it should be. But the use of a planetary
combination enables us to cause the motions of two independent trains to
converge, and unite in producing a single resultant rotation. This may be
done in two ways; each of the two independent trains may drive one
sun-wheel, thus determining the motion of the train-arm; or, the train-arm
may be driven by one of them, and the first sun-wheel by the other; then
the motion of the second sun-wheel is the resultant. Under these
circumstances the ratio of the resultant velocity to that of either
independent train is not invariable, since it may be affected by a change
in the velocity of the other one. To illustrate our meaning, we give two
examples of arrangements of this nature. The first is Robinson's
rope-making machine, Fig. 32. The bobbins upon which the strands composing
the rope are wound turn freely in bearings in the frames, G, G, and these
frames turn in bearings in the disk, H, and the three-armed frame or
spider, K, both of which are secured to the central shaft, S. Each
bobbin-frame is provided with a pinion, _a_, and these three pinions
engage with the annular wheel, A. This wheel has no shaft, but is carried
and kept in position by t