The steam engine familiarly explained and illustrated; with an historical sketch of its invention and progressive improvement; its applications to navigation and railways; with plain maxims for railway speculators. By the Rev. Dionysius Lardner ... With additions and notes by James Renwick ...

1-02 THE STEAM ENGINE.
highest tooth of the sun wheel; as the planet wheel passes
hfiom the highest position, its teeth drive those of the sun
wheeli before them, and when it comes into the lowest position, the highest tooth of the planet wheel is engaged with
fhe lowest of the sun wheel: but then half of the sun wheel
has rYolled off the planet wheel, and, therefore, the tooth
which was engaged with it in its higher position, must now
ie- distant from it by half the circumference of the wheel,
and- must, therefore, be again in the highest position, so that,
while the planet wheel has been carried from the top to the
gottom, the sun wheel has made a complete revolution. A
little reflection, however, on the nature of the motion, will_ender this plainer than any description can. This advantagie of giving an increased velocity, may be obtained also'
1y the simple crank, by placing toothed wheels on its axle.
Independently of the greater expense attending the construction of the sun and planet wheel, its liability to go out of
6rder, and the rapid wear of the teeth, and other objections,
rendered it decidedly inferior to the crank, which has now
Dntirely superseded it.
(58.)" Whether the simple crank or the sun and planet
wheel: be used, there still remains a difficulty of a peculiar
iaature attending the continuance of the rotatory motion.
There are two positions in which the engine can give no
motion whatever to the crank. These are when the end of
tahe beam, the axle of the crank, and the pivot which joins
the connector with the crank, are in the same straight line.
This will be easily understood. Suppose the beam, coni6eetor, and crank to assume the position represented in fig.
15. If steam urge the piston downward, the point H and
the connector H I will be drawn directly upward. But it
must be very evident that in the present situation of the
connector H I, and the lever I K, the force which draws the
oi-nt I in the direction I K can have no effect whatever in
turning I K round the centre I, but will merely exert a pressu're on the axle or pivots of the wheel.