577. These principles have many applications in the mechanical arts;
we shall mention two of them. The first is to the governor-balls of a
steam-engine; the second is to the process of sugar refining.
An engine which turns a number of machines in a factory should work
uniformly. Irregularities of motion may be productive of loss and
various inconveniences. An engine would work irregularly either from
variation in the production of steam, or from the demands upon the
power being lessened or increased. Even if the first of these sources
of irregularity could be avoided by care, it is clear that the second
could not. Some machines in the mill are occasionally stopped, others
occasionally set in motion, and the engine generally tends to go faster
the less it has to do. It is therefore necessary to provide means by
which the speed shall be restrained within narrow limits, and it is
obviously desirable that the contrivance used for this purpose should
be self-acting. We must, therefore, have some arrangement which shall
admit more steam to the cylinder when the engine is moving too slowly,
and less steam when it is moving too quickly. The valve which is to
regulate this must be worked by some agent which depends upon the
velocity of the engine; this at once points to circular motion because
the force acting on the revolving body depends upon its velocity. Such
was the train of reasoning which led to the happy invention of the
governor-balls: these are shown in Fig. 79.
[Illustration: FIG. 79.]
A B is a vertical spindle which is turned by the engine. P P is a piece
firmly attached to the spindle and turning with it. P W, P W are arms
terminating in weights W W; these are balls of iron, generally very
massive: the arms are free to turn round pins at P P. At Q Q links are
placed, attached to another piece R R, which is free to slide up and
down the spindle. When A B rotates, W and W are carried round, and
therefore fly outwards from the spindle; to do this they must evidently
pull the piece R R up the shaft. We can easily imagine an arrangement
by which R R shall be made to shut or open the steam-valve according
as it ascends or descends. The problem is then solved, for if the
engine begin to go too rapidly, the balls fly out further just as they
did in Fig. 74: this movement raises the piece R R, which diminishes
the supply of steam, and consequently checks the speed. On the other
hand, when the engine works too slowly, the balls fall in towards the
spindle, the piece R R descends, the valve is opened, and a greater
supply of steam is admitted. The objection to this governor is that
though it moderates, it does not completely check irregularity. There
are other governors occasionally employed which depend also on circular
motion; some of these are more sensitive than the governor-balls; but
they are elaborate machines, only to be employed under exceptional
circumstances.
578. The application of circular motion to sugar refining is a very
beautiful invention. To explain it I must briefly describe the process
of refining.
The raw sugar is dissolved in water, and the solution is purified by
straining and by filtration through animal charcoal. The syrup is
then boiled. In order to preserve the colour of the sugar, and to
prevent loss, this boiling is conducted _in vacuo_, as by this means
the temperature required is much less than would be necessary with the
ordinary atmospheric pressure.
The evaporation having been completed, crystals of sugar form
throughout the mass of syrup. To separate these crystals from the
liquor which surrounds them, the aid of circular motion force is
called in. A mass of the mixture is placed in a large iron tub, the
sides of which are perforated with small holes. The tub is then made
to rotate with prodigious velocity; its contents instantly fly off
to the circumference, the liquid portions find an exit through the
perforations in the sides, but the crystals are left behind. A little
clear syrup is then sprinkled over the sugar while still rotating: this
washes from the crystals the last traces of the coloured liquid, and
passes out through the holes; when the motion ceases, the inside of
the tub contains a layer of perfectly pure white sugar, some inches in
thickness, ready for the market.
579. Circular motion is peculiarly fitted for this purpose; each
particle of liquid strives to get as far away from the axis as
possible. The action on the sugar is very different from what it would
have been had the mass been subjected to pressure by a screw-press or
similar contrivance; the particles immediately acted on would then have
to transmit the pressure to those within; and the consequence would
be that while the crystals of sugar on the outside would be crushed
and destroyed, the water would only be very imperfectly driven from
the interior: for it could lurk in the interstices of the sugar, which
remain notwithstanding the pressure.
580. But with circular motion the water must go, not because it is
pushed by the crystals, but because of its own inertia; and it can be
perfectly expelled by a velocity of rotation less than that which would
be necessary to produce sufficient pressure to make the crystals injure
each other.