revolutions to be made per minute, and reserve the product for a divisor;
multiply the number of _actual_ horses power to be transmitted by 240, and
divide the product by the above divisor, which will give the strength. If
the pitch be given to find the breadth, divide the above strength by the
square of the pitch in inches; or if the breadth be given, then to find the
pitch divide the strength by the breadth in inches, and extract the square
root of the quotient, which is the proper pitch in inches. The length of
the teeth is usually about 5/8ths of the pitch. Pinions to work
satisfactorily should not have less than 30 or 40 teeth, and where the
speed exceeds 220 feet in the minute, the teeth of the larger wheel should
be of wood, made a little thicker, to keep the strength unimpaired.

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: