as been desirable, as in the case of marine engines
employed to drive the screw propeller without intermediate gearing, India
rubber discs, resting on a perforated metal plate, are now generally
adopted; but the India rubber should be very thick, and the guards employed
to keep the discs down should be of the same diameter as the discs
themselves.

219. _Q._--Can you suggest any eligible method of enabling condensing
engines to work satisfactorily at a high rate of speed?

_A._--The most feasible way of enabling condensing engines to work
satisfactorily at a high speed, appears to lie in the application of
balance weights to the engine, so as to balance the momentum of its moving
parts, and the engine must also be made very strong and rigid. It appears
to be advisable to perform the condensation partly in the air pump, instead
of altogether in the condenser, as a better vacuum and a superior action of
the air pump valves will thus be obtained. Engines constructed upon this
plan may be driven at four times the speed of common engines, whereby an
engine of large power may be purchased for a very moderate price, and be
capable of being put into a very small compass; while the motion, from
being more equable, will be better adapted for most purposes for which a
rotary motion is required. Even for pumping mines and blowing iron
furnaces, engines of this kind appear likely to come into use, for they are
more suitable than other engines for driving the centrifugal pump, which in
many cases appears likely to supersede other kinds of pumps for lifting
water; and they are also conveniently applicable to the driving of fans,
which, when so arranged that the air condensed by one fan is employed to
feed another, and so on through a series of 4 or 5, have succeeded in
forcing air into a furnace with a pressure of 2-1/2 lbs. on the square
inch, and with a far steadier flow than can be obtained by a blast engine
with any conceivable kind of compensating apparatus. They are equally
applicable if blast cylinders be employed.

220. _Q._--Then, if by this modification of the engine you enable it to
work at four times the speed, you also enable it to exert four times the
power?

_A._--Yes; always supposing it to be fully supplied with steam. The nominal
power of this new species of engine can readily be ascertained by taking
into account the speed of the piston, and this is taken into account by the
Admiralty rule for power.

221. _Q._