the end of another
shaft carrying pinions 6 and 4. Pinion 6 slides up and down this shaft,
which is square at this point, but round inside the _loose_ pinion 4.
Pinions 2 and 3 are keyed to a square secondary shaft, and are
respectively always in gear with 1 and 4; but 5 can be slid backwards
and forwards so as to engage or disengage with 6. In the illustration no
gear is "in." If the engine is working, 1 revolves 2, 2 turns 3, and 3
revolves 4 idly on its shaft.

[Illustration: FIG. 48.--The gear-box of a motor car.]

To get the lowest, or "first," speed the driver moves his lever and
slides 5 into gear with 6. The transmission then is: 1 turns 2, 2 turns
5, 5 turns 6, 6 turns the propeller shaft through the universal joint.
For the second speed, 5 and 6 are disengaged, and 6 is moved up the
page, as it were, till projections on it interlock with slots in 4; thus
driving 1, 2, 3, 4, shaft. For the third, or "solid," speed, 6 is pulled
down into connection with 1, and couples the engine shaft direct to the
propeller shaft.

The "reverse" is accomplished by raising a long pinion, 7, which lies in
the gear-box under 5 and 6. The drive then is 1, 2, 5, 7, 6. There being
an odd number of pinions now engaged, the propeller shaft turns in the
reverse direction to that of the engine shaft.

[Illustration: FIG. 49.]

THE COMPENSATING GEAR.

Every axle of a railway train carries a wheel at each end, rigidly
attached to it. When rounding a corner the outside wheel has further to
travel than the other, and consequently one or both wheels must slip.
The curves are made so gentle, however, that the amount of slip is very
small. But with a traction-engine, motor car, or tricycle the case is
different, for all have to describe circles of very small diameter in
proportion to the length of the vehicle. Therefore in every case a
_compensating gear_ is fitted, to allow the wheels to turn at different
speeds, while permitting them both to drive. Fig. 49 is an exaggerated
sketch of the gear. The axles of the moving wheels turn inside tubes
attached to the springs and a central casing (not shown), and terminate
in large bevel-wheels, C and D. Between these are small bevels mounted
on a shaft supported by the driving drum. If the latter be rotated, the
bevels would turn C and D at equal speeds, assuming that both axles
revolve without friction in their bearings. We will suppose that the
drum is turned 50 times a minute. Now, if one w