MARINE ENGINES.

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times as much, or a proportion of 35 to 1 would be required; thatis to say, the starting wheel would require to make 35 revolutionsfor one of the shaft on which the lifting arm for raising and lower-ing the link is fitted. Of course for small power a valve should bearranged to take the steam pressure off the back, when the slidevalve is moved by hand, thus lessening the labour in reversing thegear.

Connecting rod —'The thrust and pull on the connecting rodreceive the full force exerted on the piston, and the same rule holdsgood as for the piston rod, namely, 2240 lbs. for compressive stressand 4000 for tensional strain. This gives the number of squareinches of area for the ends of the rod, an allowance of one-fourth ofan inch in diameter for every foot in length gives the diameter atthe middle. The thickness of caps, when so fitted, is one-half of thediameter of the rod. The combined sectional area of the bolts atthe bottom of the thread is found by dividing the total pressure onpiston (i.e. steam and vacuum) by 4000. When straps are used,the combined area at the brasses and key-ways must equal the areaof the rod at end. The breadth of jibs and cotters equals thediameter of the rod, and the thickness of the key is found by mul-tiplying the diameter of rod by *31; the thickness of brasses atends is found by dividing the diameter of the crank pin by 8, andfor the side one-half will suffice. The brasses are planed out andlined with white metal, the proportion being for large sizes 1 inchof brass surface all round with a mid strip left.

Main cranked shaft .—The same rule holds good as regards thediameter of a shaft with cranks forged on in one piece, and plaincranks shrunk on a straight shaft. When the cranks of two enginesare placed at right angles to each other, the shaft should have thesame area as a single engine having the same force exerted on thepiston, the length of the crank from centre to centre being identical.To find the diameter of the shaft, multiply the length of the crankin inches by the total pressure on the piston (i.e. steam and vacuum),and divide the sum by 1206; the cube root of the quotient will bethe diameter of the shaft. The collars at the bearings for largediameters may be 1 inch. The length of the two outside journalsis generally equal to twice the diameter of the shaft at the Journal.The middle bearing is made longer according to the distancebetween the frames for supporting the shaft. The crank pin is thesame diameter as the Journal; the distance between the jaws of the