an enormous number of tiny stones. Now, escaping
high-pressure steam moves very fast indeed. To give figures, if it
enters the small end of a De Laval nozzle at 200 lbs. per square inch,
it will leave the big end at a velocity of 48 miles per _minute_--that
is, at a speed which would take it right round the world in 8-1/2 hours!
The wheel itself would not move at more than about one-third of this
speed as a maximum.[7] But even so, it may make as many as 30,000
revolutions per minute. A mechanical difficulty is now
encountered--namely, that arising from vibration. No matter how
carefully the turbine wheel may be balanced, it is practically
impossible to make its centre of gravity coincide exactly with the
central point of the shaft; in other words, the wheel will be a
bit--perhaps only a tiny fraction of an ounce--heavier on one side than
the other. This want of truth causes vibration, which, at the high speed
mentioned, would cause the shaft to knock the bearings in which it
revolves to pieces, if--and this is the point--those bearings were close
to the wheel M. de Laval mounted the wheel on a shaft long enough
between the bearings to "whip," or bend a little, and the difficulty was
surmounted.

The normal speed of the turbine wheel is too high for direct driving of
some machinery, so it is reduced by means of gearing. To dynamos, pumps,
and air-fans it is often coupled direct.

THE PARSONS TURBINE.

At the grand naval review held in 1897 in honour of Queen Victoria's
diamond jubilee, one of the most noteworthy sights was the little
_Turbinia_ of 44-1/2 tons burthen, which darted about among the floating
forts at a speed much surpassing that of the fastest "destroyer." Inside
the nimble little craft were engines developing 2,000 horse power,
without any of the clank and vibration which usually reigns in the
engine-room of a high-speed vessel. The _Turbinia_ was the first
turbine-driven boat, and as such, even apart from her extraordinary
pace, she attracted great attention. Since 1897 the Parsons turbine has
been installed on many ships, including several men-of-war, and it seems
probable that the time is not far distant when reciprocating engines
will be abandoned on all high-speed craft.

DESCRIPTION OF THE PARSONS TURBINE.

[Illustration: FIG. 37.--Section of a Parsons turbine.]

The essential parts of a Parsons turbine are:--(1) The shaft, on which
is mounted (2) the drum; (3) the cylindrical casing insi