etal to
the action of centrifugal force under exclusion of air, producing not
only the most minute division of the particles, but also a vacuum, both
favorable conditions for obtaining a dense metal casting.

Most of the devices for drying steam come under this head. Such are
those in which the steam with the water in suspension is forced to take
a circular path, by which the water is hurled by centrifugal force
against the concave side of the passage and passes back to the water in
the boiler.

SPEED.--The centrifugal force of a revolving particle varies, as we
have seen, as the square of the angular velocity, so that the effort
has been to obtain as high a number of revolutions per minute as was
consistent with safety and with the principle of the machine. For
example, creamers which are small and light make 4,000 revolutions per
minute, though the latest styles run much more slowly. Driers and sugar
machines vary from 600 to 2,000, while on the other hand the necessity
of keeping the mercury from hurling off in an amalgamator prevents its
turning more rapidly than sixty or eighty times a minute.

However, speed in another sense, the speed with which the operation is
performed, is what especially characterizes centrifugal extractors. In
this particular a contrast between the old methods and the new is
impressive. Under the action of gravity, cream rises to the milk's
surface, but compare the hours necessary for this to the almost
instantaneous separation in a centrifugal creamer. The sugar
manufacturer trusted to gravity to drain the sirup from his crystals,
but the operation was long and at best imperfect. An average sugar
centrifugal will separate 600 pounds of magma perfectly in three
minutes. Gold quartz which formerly could not pay for its mining is now
making its owners' fortunes. It is boasted by a Southern company that
whereas they were by old methods making twenty-five _cents_ per ton of
gold quartz, they now by the use of the latest amalgamator make
twenty-five _dollars_. Centrifugal force, as applied in extractors, has
opened up new industries and enlarged old ones, has lowered prices and
added to our comforts, and centrifugal extractors may well command, as
they do, the admiration of all as wonderful examples of the way in
which this busy age economizes time.

* * * * *

A NEW TYPE OF RAILWAY CAR.

[Illustration: Fig. 1.--CAR WITH LATERAL PASSAGEWAYS.]

Figs.