MACHINERY — BOAT AND SHIP BUILDING. 51 



are the entire apparatus of a Persian wheel, sugarcane and oil mills, 

 pulleys, windlasses, tilt hammers and water-wheels. In cog-wheels 

 used in machinery set up at a distance from a workshop where 

 repairs can be effected, the cogs are best made of some hard, 

 tough wood, since the only part of such wheels that is constantly 

 breaking consists of the teeth, and broken wooden cogs can be 

 at once replaced from a lot kept in stock for the purpose. All 

 parts subject to friction should be made of the hardest and toughest 

 woods obtainable. For the crushers of sugar and oil mills the 

 wood sliould also be as heavy as possible, like Hardwickia binata, 

 khair, iron wood, babul, Schleichera trijuga, Mesuaferrea. The best 

 woods for axle trees are such as are hard and tough, and have 

 anastomosed fibres, but without knots, such as babul, sissu, &c. 



In many cases wood is used in fixing machinery- In order 

 to stand the constant jar and heavy strains while the machinery is 

 working, the wood should be very hard and strong. 



AeTIOLE 5. TiMBKB USED IN BOAT AND SHIP BUILDINO. 



More care has to be exercised in selecting wood for ship and 

 boat building than for almost any other purpose, firstly, on 

 account of the extremely unfavourable conditions in which the 

 wood is used for its durability; secondly, on account of the general- 

 ly peculiar shape of the diflferent structures ; thirdly, on account of 

 the enormous strains sea-going boats have to withstand ; and , 

 lastly, on account of the serious risks attending a breakdown of 

 any portion of a ship or boat. 



The main differences in shape between sea-going boats and 

 those intended for traflSc on rivers are — (1), that the former 

 are shorter and narrower in proportion to their depth, (2), that 

 they have a keel, whereas river boats are flat-bottomed, and (3), 

 that the former have curved sides exhibiting every degree ot curva- 

 ture, whereas the lines of the others are comparatively straight. 



Timber for boat and ship building, besides being as sound and 

 durable as possible, must be quite free from faults, must be strong 

 and elastic, and must be of the right weight, shape and dimensions. 



To give a ship stability the centre of gravity must be precisely 

 at a certain height, and hence the importance of using heavier wood 

 in building the sides than the deck, and the necessity of having the 

 masts light, but at the same time as strong and elastic as possible. 



As regards shape the ribs or framework of a ship or sea-going 

 boat must consist of naturally curved pieces (compass timber, 

 crooks, bends), the curvature being measured by the proportion 

 between the length of the chord and the height of the arc. The 



