CAUSES OF DECAY AND DESTRUCTION OF TIMBER 261 



the timber caps and pile heads down to original ground 

 level are in many cases quite decayed ; in some the timber 

 crumbles away beneath the fingers, being quite dry and 

 brittle, and in other cases wet and soft ; those portions of 

 the piles below the original surface are quite sound ; the 

 portions of pile heads surrounded by sand are the most 

 decayed. It is known that made-up ground affects timber 

 much more than natural ground. Ashes are about the 

 worst material in which to place unpreserved wood, and 

 sand filling often seriously affects timber. The only safe 

 material in which to bed timber when that has to be done 

 is clay, as this can be made impervious to moisture and 

 air. 



Timber bridges, formerly much useU on the United 

 States railways, which when exposed to the weather lasted 

 only seven or eight years, when roofed over would last 

 thirty years or more, and the importance of some shelter 

 or protection for timber and of thorough ventilation, 

 especially in a moist climate, cannot be overstated for 

 prolonging its life. 



It is in those portions of timber structures most affected 

 by variations of temperature, sometimes dry, sometimes 

 wet and often damp, and in situations where there is no 

 current of air, that decay first sets in. 



In exposed timber work, such as planking or decking of 

 quays, dirt lies along the seams and joinings and keeps the 

 timber in a moist condition, considerably shortening its 

 life ; the joists on which the decking rests are liable to 

 decay more quickly than other portions of the work, being 

 shut out from light and air and kept damp ; those portions 

 between high and low water level which are always wet or 

 damp last longer than the higher portions ; in the top 

 bracing above tide level, even where there is no planking 

 on the top, the upper surface of the beams, particularly the 



