366. Q.—­How do you find the diameter of the main centre when proportioned according to this rule?

A.—­The diameter of the main centre may be found by multiplying 0.0367 times the square root of the pressure upon the piston in lbs. per square inch, by the diameter of the cylinder in inches, which will give the diameter of the main centre journal 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 as to distribute the pressure, and prevent the disposition to heat which would otherwise exist.  The cross head journals