in opposite directions with equal velocities, which gives _n_/_m_ = -1;
but when A is fixed and T revolves, we have _m'_ = 0, whence in the
general formula

n' - a
------ = -1, or n' = 2 a;
-a

which means, being interpreted, that F makes two rotations about its
axis during one revolution of T, and in the same direction. Again, let
A and F be equal in the 3-wheel train, Fig. 16, the former being fixed
as before. In this case we have:

n
--- = 1, m' = 0, which gives
m

n' - a
------- = 1, [therefore] n' = 0;
-a

that is to say, the wheel F, which now evidently has a motion of
circular translation, does not rotate at all about its axis during the
revolution of the train-arm.

[Illustration: PLANETARY WHEEL TRAINS. Fig. 16]

All this is perfectly consistent, clearly, with the hypothesis that
the motion of circular translation is a simple one, and the motion of
revolution about a fixed axis is a compound one.