of the cylinder should be about 1/16th its diameter, which, with a pressure of steam of 80 lbs. upon the square inch, will occasion a strain of 640 lbs. upon the square inch of section of the metal; and the thickness of the metal of the trunnion bearings of high pressure oscillating engines should be 1/13th of the diameter of the cylinder.  The strength, however, is not the sole consideration in proportioning cylinders, for they must be made of a certain thickness, however small the pressure is within them, that they may not be too fragile, and will stand boring.  While, also, an engine of 40 inches diameter would be about one inch thick, the thickness would not be quite two inches in an 80 inch cylinder.  In fact there will be a small constant added to the thickness for all diameters, which will be relatively larger the smaller the cylinders become.  In the cylinders of Penn’s 12 horse power engines, the diameter of cylinder being 21-1/2 inches, the thickness 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.