RELIEF VALVES AND DRAIN COCKS.         201

a rigid shaft and a flexible hull the result is an enormous
amount of friction in the bearings, which consumes power, and
often causes the bearings "to fire." Scarcely a month passes
that we do not hear of some steamer breaking a main shaft,
often to the danger of life and property, and this in many
instances may be traced to a want of trueness in the line of the
bearings, due to the vessel having warped from uneven stowage)
or from having encountered heavy weather.

The coupling consists of a projection formed on the end of the
paddle shaft, P S, which is part of a ball, the centre of this
projection being formed into a blunt point at, <i>p</i>. This point
rests hard against the crank shaft, C S, and transmits any
thrust along the line of shafting. The outside of the projection
is clasped by the coupling ring, C, turned to fit the ball joint.
This ring, C, is made in halves (as may be seen by the end
view), and is secured to the crank shaft by means of the driving
bolts, D B. The concave portion of this ring takes any pull
that may come on the shaft along the line of shafting. The
ends of the driving bolts, D B, project as shown into holes
in the paddle shaft, P S, and thus act as drivers. These
projecting pins are made 2 1/2 times the diameter of the bolts
usually employed for main-shaft coupling flanges. These pins
are slightly barrel-shaped in form, and made an easy fit for the
holes in which they work. The holes are lined with hard steel
bushes, S B, while the pins are case-hardened to prevent chafing
and wearing away. It will be observed that there is a small
space left clear between the paddle shaft flange and the coupling
ring, C, to permit of perfect up and down or side play, or
un-linement between the crank and paddle shafts for the reasons
already mentioned.

<i>Relief Valves</i>. - On referring to the perspective view of the
engines, it will be seen that relief valves, R V, are fitted not
only to the front and back of the high- and low-pressure cylinders,
but also to the back of the low-pressure slide valve casing, for
the purpose of preventing damage to these parts through water
gathering in them. These relief valves consist of ordinary mush-
room valves, held down by strong spiral springs, and adjusted to
any desired pressure by hand wheels and screws, as shown.

<i>Drain Cocks</i>, D C, are fitted to the back of tho high-pressure
slide valve casing, and to the bottoms of both cylinders. The
pipes leading from them are all connected to the condenser.
These cocks are opened before starting the engines, so as to clear
away any water that may have resulted from condensation of
steam, and also when the engines have to be stopped for any
length of time.