94 



PROPERTIES OF Wool). 



strength. If a piece of wood is subject to great pressure, as 

 in wheel-spokes or ladder-rungs, it should be split or cloven 

 from the log, as, by sawing or hewing, many iibres are cut, 

 whilst by splitting, all the iibres remain in their natural length. 



Evenness of grain in the annual rings and approximately 

 vertical fibres denote strong wood. Any interruption of the 

 straightness of grain, caused especially by enclosed branches, 

 reduces the strength of the beam considerably. 



Since the elasticity of a beam is ascribed correctly to the 

 amount of liguin it contains, exposure to light and heat 



400. 



75. 



70. 



Fig. -A3. Ratios of the strength of different beams of the same section in area, 

 but of different shapes. 



during the life of a tree must be favourable to the strength 

 of its wood, for Cieslar has shown that the cell-walls become 

 more lignilied the more light a tree receives. On the other 

 hand, wood, especially that of suppressed poles, grown in a 

 dense crop is tough, but less elastic and strong. Practical 

 experience confirms this statement, for spruce poles grown in 

 open woods belonging to peasants, called "white stems" on 

 account of the lichens that cover them, are more durable and 

 elastic than " red poles " taken from thinnings in a dense 

 wood. The opinion of timber-merchants that mountain-wood 

 is more elastic than valley-wood is also partly true. 



