356. *Q.*—What was Mr. Watt’s
rule for the pitch of wheels?

*A.*—Multiply five times the diameter
of the larger wheel by the diameter of the smaller,
and extract the fourth root of the product, which is
the pitch.

STRENGTH OF MARINE AND LOCOMOTIVE ENGINES.

357. *Q.*—Cannot you give some rules
of strength which will be applicable whatever pressure
may be employed?

*A.*—In the rules already given, the
effective pressure may be reckoned at from 18 to 20
lbs. upon every square inch of the piston, as is usual
in land engines; and if the pressure upon every square
inch of the piston be made twice greater, the dimensions
must just be those proper for an engine of twice the
area of piston. It will not be difficult, however,
to introduce the pressure into the rules as an element
of the computation, whereby the result will be applicable
both to high and low pressure engines.

358. *Q.*—Will you apply this mode
of computation to a marine engine, and first find
the diameter of the piston rod?

*A.*—The diameter of the piston rod
may be found by multiplying the diameter of the cylinder
in inches, by the square root of the pressure on the
piston in lbs. per square inch, and dividing by 50,
which makes the strain 1/7th of the elastic force.

359. *Q.*—What will be the rule for
the connecting rod, supposing it to be of malleable
iron?

*A.*—The diameter of the connecting
rod at the ends, may be found by multiplying 0.019
times the square root of the pressure on the piston
in lbs. per square inch by the diameter of the cylinder
in inches; and the diameter of the connecting rod
in the middle may be found by the following rule:—to
0.0035 times the length of the connecting rod in inches,
add 1, and multiply the sum by 0.019 times the square
root of the pressure on the piston in lbs. per square
inch, multiplied by the diameter of the cylinder in
inches. The strain is equal to 1/6th of the elastic
force.

360. *Q.*—How will you find the diameter
of the cylinder side rods of a marine engine?

*A.*—The diameter of the cylinder
side rods at the ends may be found by multiplying
0.0129 times the square root of the pressure on the
piston in lbs. per square inch by the diameter of
the cylinder; and the diameter of the cylinder side
rods at the middle is found by the following rule:—to
0.0035 times the length of the rod in inches, add 1,
and multiply the sum by 0.0129 times the square root
of the pressure on the piston in lbs. per square inch,
multiplied by the diameter of the cylinder in inches;
the product is the diameter of each side rod at the
centre in inches. The strain upon the side rods
is by these rules equal to 1/6th of the elastic force.