circles being so very much smaller--the
number of revolutions effected in a given time is much greater. Thus a
steam jet turning a pendant turbine--dipping into the middle of the
whirlpool and carrying paddles--at an enormously high speed may be
made to impart motion to the water in a circular tank (or, if desired,
to the tank itself) at a very much slower rate; the amount of the
reduction, of course, depending mainly on the ratio between the
diameter of the tank and the length of the small paddles at the centre
setting the liquid in motion.

For special purposes it is best to substitute a spherical for an
ordinary circular tank and the size may be greatly diminished by using
mercury instead of water. The sphere is complete, excepting for a
small aperture at the top for the admission of the steel shaft of the
steam-driven turbine. No matter how high may be the speed, the liquid
cannot be thrown out from a spherical revolving receptacle constructed
in this way. Moreover, the mercury acts not only as a transmitter of
the power from the turbine to the purpose for which it is wanted, but
also as a governor. Whenever the speed becomes so great as to throw
the liquid entirely into the sides of the sphere--so that the shaft
and paddles are running free of contact with it in the middle--the
machine slows down, and it cannot again attain full speed until the
same conditions recur.

The rate of speed which may be worked up to as a maximum is determined
by the position of the paddle-wheel, which is adjustable and floats
upon the liquid although controlled in its circular motion by the
shaft which passes through a square aperture in it and also a sleeve
extending upward from it. The duty of the latter is to economise steam
by cutting off the jet as soon as, by its rapidity of motion, the
paddle-wheel has thrown the mercury to the sides to such an extent as
to sink to a certain level in the centre.

Cheap motors coupled with cheap dynamos will, in the twentieth
century, go far towards lightening the labours of millions whose toil
is at present far too much of a mere mechanical nature. The dynamo
itself, however, requires to be greatly reduced in first cost.
Particularly it is necessary that the expense involved in drawing the
wire, insulating it, and winding machines with it, should be
diminished. This will no doubt be partly accomplished by the
electrolytic producers of copper when once they get properly started
on methods of