urnal in inches when of malleable iron, and
the length of the main centre journal should be 1-1/2 times its diameter;
the strain upon the main centre journal in ordinary working will be about
1/2 the elastic force.

367. _Q._--What are the proper dimensions of the gibs and cutters of an
engine?

_A._--The depth of gibs and cutters for attaching the piston rod to the
cross head, is 0.0358 times the cube root of the pressure of the steam on
the piston in lbs. per square inch, multiplied by the diameter of the
cylinder; and the thickness of the gibs and cutters is 0.007 times the cube
root of the pressure on the piston in lbs. per square inch, multiplied by
the diameter of its cylinder. The depth of the cutter through the piston is
0.017 times the square root of the pressure on the piston in lbs. per
square inch, multiplied by the diameter of the cylinder in inches; and the
thickness of the cutter through the piston is 0.007 times the square root
of the pressure on the piston in lbs. per square inch, multiplied by the
diameter of the cylinder.

368. _Q._--Are not some of the parts of an engine constructed according to
these rules too weak, when compared with the other parts?

_A._--It is obvious, from the varying proportions subsisting in the
different parts of the engine between the strain and the elastic force,
that in engines proportioned by these rules--which represent nevertheless
the average practice of the best constructors--some of the parts must
possess a considerable excess of strength over other parts, and it appears
expedient that this disparity should be diminished, which may best be done
by increasing the strength of the parts which are weakest; inasmuch as the
frequent fracture of some of the parts shows that the dimensions at present
adopted for those parts are scarcely sufficient, unless the iron of which
they are made is of the best quality. At the same time it is quite certain,
that engines proportioned by these rules will work satisfactorily where
good materials are employed; but it is important to know in what parts good
materials and larger dimensions are the most indispensable. In many of the
parts, moreover, it is necessary that the dimensions should be proportioned
to meet the wear and the tendency to heat, instead of being merely
proportioned to obtain the necessary strength; and the crank pin is one of
the parts which requires to be large in diameter, and as long as possible
in the bearing, so