126 INDT'STEIAL USES OF WOOD. 



between ships intended for the sea, and fresh -Avuter barp^cs : the 

 former are comparatively short compared with their breadth, with 

 keels which run straight from end to end of the ship ; whilst all 

 the other lines are of different degrees of curvature. 



This curved shape is given to ships by means of ribs, which 

 are partly made by joining difterent pieces of wood, but also by 

 using curved pieces. 



Fresh-water barges have no keels, but a broad flat bottom on 

 which the knee-pieces are fastened at a sharp angle, so that 

 the straight line is much more frequent in their construction 

 than in that of ships. The chief strength in ships consists 

 in the ribs which are very close together, the outer planking 

 being less important ; in barges the ribs are much further apart, 

 and the planking is of greater importance. 



The demands on wood for ship and boat-building depends on 

 species of wood, its quality, shape and strength. 



2. Species and (Jtialitij. 



Oakwood is the principal material used in ship-building, and 

 nearly the whole framework of wooden ships, boats and barges is 

 made of it. All oakwood is not, however, suitable for the purpose ; 

 for there is much inferior oakwood which is worse than several 

 other timbers. 



Durability and strength are the principal requisites in ship- 

 building timber ; and to ensure these qualities in oakwood, only 

 broad-zoned timber should be chosen, with annual zones up 

 to 7 to 8 millimeters, say I inch, with a narrow zone of 

 fine pores. The wood should be light-coloured rather than 

 dark-coloured when freshly cut : in any case, of uniform 

 colour tliroughout, as long-tibred as possible, tough and with a 

 strong fresh odour of tannic acid. 



Inferior oakwood has narrow annual zones, comparatively 

 broad porous zones and wide pores, is short-fibred and brittle, 

 and darkish coloured, streaky, or reddish in colour and only 

 slightly scented. 



It is evident that not only the very best kinds of oak-timber 

 are used in ship-building, the ship-builder knows the parts in 

 the ship where the less valuable qualities may be used ; but it is 

 also evident that there is a limit to the use of inferior oakwood 



