CAUSES OF DECAY AND DESTRUCTLON 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 used 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 
