of the metal is 9/16ths: in Penn's 40 inch cylinders,
the thickness is 1 inch, and in the engines of the Ripon, Pottinger, and
Indus, by Messrs. Miller, Ravenhill and Co., with cylinders 76 inches
diameter, the thickness of the metal is 1-11/16. These are all oscillating
engines.

343. _Q._--What is the proportion of the piston rod?

_A._--The diameter of the piston rod is usually made 1/10th of the diameter
of the cylinder, or the sectional area of the piston rod is 1/100th of the
area of the cylinder. This proportion, however, is not applicable to
locomotive, or even fast moving marine engines. In locomotive engines the
piston rod is made 1/7th of the diameter of the cylinder, and it is obvious
that where the pressure on the piston is great, the piston rod must be
larger than when the pressure on the piston is small.

344. _Q._--What are the proper dimensions of the main links of a land beam
engine?

_A._--The sectional area of the main links in land beam engines is 1/113th
of the area of the cylinder, and the length of the main links is usually
half the length of the stroke.

345. _Q._--What are the dimensions of the connecting rod of a land engine?

_A._--In land engines the connecting rod is usually of cast iron with a
cruciform section: the breadth across the arms of the cross is about 1/20th
of the length of the rod, the sectional area at the centre 1/28th of the
area of the cylinder, and at the ends 1/35th of the area of the cylinder:
the length of the rod is usually 3-1/2 times the length of the stroke. It
is preferable, however, to make the connecting rod of malleable iron, and
then the dimensions will be those proper for marine engines.

346. _Q._--What was Mr. Watt's rule for the connecting rod?

_A._--Some of his connecting rods were of iron and some of wood. To
determine the thickness when of wood, multiply the square of the diameter
of the cylinder in inches by the length of the stroke in feet, and divide
the product by 24. Extract the fourth root of the quotient, which is the
thickness in inches. For iron the rule is the same, only the divisor was
57.6 instead of 24.

347. _Q._--What are the dimensions of the end studs of a land engine beam?

_A._--In low pressure engines the diameter of the end studs of the engine
beam are usually made 1/9th of the diameter of the cylinder when of cast
iron, and 1/10th when of wrought iron, which gives a load with low steam of
about 500 lbs. per circular inch