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:—­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.