IRON SHIPBUILDING.

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elasticity, and if not uniformly elastic, the part having the greatestmeasure of elasticity is sure to separate from that portion of thestructure which is the most rigid. In iron steamships it is usual tomake the length between the perpendiculars from seven to ninetimes the breadth of beam, Some builders, however, have gone sofar as to increase that proportion to ten and three-quarters; andwhen such ships are properly constructed, locating the greateststrength where it is most required, namely, at midships, they havebeen found to answer without showing signs of weakness.

All vessels may be practically considered as hollow beams;the deck is the top web, and the bottom of the ship the lower web.In all scientifically constructed beams of the parallel kind, thegreatest sectional area of the webs is at the centre, therefore at thebottom of a ship amidships the sectional area oi the plating shouldbe much greater than at the ends, and the deck should be platedor strengthened with Stringers and diagonals. In fact, to make itresemble a hollow beam, some plate the deck all over: this plan mayanswer with corrugated plates, but plain plating is not so suitable.A plated deck, however, can be covered with a wooden one; thissecures as good foothold as in ordinary ships, while the comfortafforded in all weathers far exceeds that of plain iron decks. Thelatter are very cold for the feet in winter, too hot in summer andintolerably so in tropical climes; and the foothold in rough weatheris very precarious, the plates becoming smooth and slippery.

The depth of hold varies from one-tenth to one-twelfth of thelength between the perpendiculars. The length between the per-pendiculars is measured on the line of deck or mid-deck for spar-decked ships taken from the after end of the stern post to the forerake, or exactly the length of the vessel’s deck from the after partof the stern post forward.

The displacement is the cubical contents of the immersed portionof the vessel; the ship resting on the water displaces a certain bulle,the weight of the water displaced being equal to the total weight ofthe vessel and all contained in it. When the draught of the shipis made it is necessary to calculate the displacement in cubic feet;the weight of a cubic foot of sea water is taken at 64 lbs., and thereare 35 cubic feet to the ton. The displacement may be calculatedapproximately by multiplying together the immersed length, depth,and breadth, and taking, according to the dass of vessel, from one-half to three-fifths of the cube thus obtained; this will be the dis-

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