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PILING. 65
offer greater résistance and attain a higher degree of exemption than do
members of the third and last group.
Elm {Ulmus) and Beech {Fagus sylvaticd) are two well-known timbers,
to which the term durable is only applicable provided the conditions be
those of total immersion or continuons dryness. The weight of elm is about
35 Ibs., and the weight of beech about 48 Ibs. per cubic foot. As regards
strength, beech has the superiority, being half as strong again as elm. The
mean ultimate transverse load on a unit beani of elm is 400 Ibs. ; that of
beech, 600 Ibs.
Pine and Fir include a number of varieties of timber, some of which,
such as pitchpine and Oregon pine, are highly serviceable to the harbour
engineer for temporary staging and dams. Their durability under exposure
to water is not very great, unless it be assisted by some treatment, as
creosoting, which also affords protection to a certain extent against insects.
These timbers must needs, however, be under constant supervision and
inspection, and it is certainly not desirable to set them in positions difficult
of access nor to place too great confidence in their capabilities of resistance.
Pitchpine {Pinus rigida) is obtained from the southern states of North
America. It is a highly resinous wood, reddish or reddish brown in colour.
The resin in its pores renders it hard and difficult to work, but also increases
its durability. The strength of pitchpine is often reduced by the practice of
“bleeding ” the growing tree, that is, tapping it for the turpentine which it
contains. Logs are obtainable from 10 to 18 inches square and up to 60 or
70 feet long. The commonest sizes for piling purposes are from 12 to 15
inches square and from 40 to 50 feet long.
Oregon Pine {Abies Douglasii) cornes from the north-west of North
America. It has a light reddish colour. It is obtainable in logs up to 20
and 24 inches square and up to 100 feet in length. It is not so strong as
pitchpine, but, affording larger sizes, is useful in certain situations.
Destruction of Timber.—The utility and value of timber being so
greatly affected by its liability to destruction and decay, it is necessary to
consider the sources of deterioration and the possibilities of their avoidance
or cure.
Insectile ravages claim first attention, as they constitute the most serious
and pressing danger to which timber piles are exposed. Woods of the utmost
durability in regard to ohemical changes sucoumb only too rapidly from
purely mechanical causes.
Teredo Navalis.—This animal, one of the most pertinacious assailants of
marine timber structures, is a member of the family Pholadidæ. It is found in
all British seas and, indeed, frequents the majority of the seaports of the world.
It has a decided preference, however, for clear salt water, and deliberately
avoids water which is muddy or sewage-polluted, or even fresh. The process
of its depredations appears to be as follows. Its eggs, drifting in the water,
adhere to any exposed woodwork against which they happen to be washed by
the sea, and there remain till ripe for hatching. On leaving its egg, the
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