BRIDGES, ETC. 575 



except that it may be partly or entirely under water. All 

 wooden bridges and works used in connection with them for 

 strengthening the banks of watercourses, sluices, weirs, booms 

 and other timber-catching apparatus on streams used for float- 

 ing, require pieces of many different shapes. Although iron 

 bridges are now becoming usual even across narrow streams, 

 and to a large extent roads are replacing water-carriage for 

 timber, yet the importance of canals for cheap traffic of heavy 

 goods is being felt more and more, so that very large quan- 

 tities of timber are required in hydraulic engineering. 



Bridges are boarded usually with beech, which gives a 

 smoother surface and is less liable to splinter than oak or 

 coniferous wood, but the considerable amount of warping and 

 shrinking of beech wood must be allowed for. 



Timber thus used i- 1 greatly to decay, so that oak- 



wood and resinous heart wood of larch and Scots pine are 

 employed generally for these purposes. In the case of works 

 for floating timber, it would be highly advantageous were the 

 best wood used, but owing to its abundance in mountainous 

 districts, and to the great cost of oak and larch, sprucewood 

 usually is employed, although its durability is small. 



Water-wheels also, for nVmrmills, sawmills and mills for other 

 purposes, should be made of oak wood, but are made usually of 

 the wood of Scots pine, larch or even spruce. The axle of a 

 water-wheel must be thoroughly sound and free from flaws; it 

 is seldom more than 18 feet long, and is made usually of the 

 wood of oak, larch, Scots pine, spruce or even beech. The 

 diameter of the axle does not depend entirely on the size of 

 the wheel, and the amount of the work to be done, but also 

 according as the spokes of the wheels are dovetailed into the 

 axle, or fastened to it tangentially. 



Iron wheel-axles rest on beech or hornbeam bearings, which 

 are supported by a strong framework of oak, Scots pine, 

 larch, etc. 



2. Fascines. 



Fascines are often used to support banks, a fascine being a 

 bundle of young stool-shoots of different species and dimen- 

 sions. Their usual length is 10 feet to 12 feet, the height to 



