In 1808 Mr. Stevenson was the discoverer of the Limnoria terebrans, that small but sure destroyer of timber structures exposed to the action of the sea, and forwarded specimens of the insect and of the timber it had destroyed to Dr. Leach, the eminent naturalist, of the British Museum, who, in 1811, announced it as a “new and highly interesting species which had been sent to him by his friend Robert Stevenson, Civil Engineer,” and assigned to it the name of Limnoria terebrans (Linnean Trans., vol. xi. p. 37, and Edinburgh Encylop?dia, vol. vii. p. 433).
The Teredo navalis, which is a larger and even more destructive enemy, is happily not so prevalent in northern seas as the Limnoria.
So impressed was Mr. Stevenson with the importance of his discovery as affecting marine engineering, and especially harbour works, that he resolved to establish a train of systematic experiments by exposing the timber of different trees to the action of salt water, and it occurred to him that no situation could be more suitable156 for such observations than the Bell Rock, where the specimens would not only be fully exposed to the sea, and free from any interference, but would be strictly watched and minutely reported on by the lightkeepers. He further conceived it proper, in the interests of the navy, to take the Admiralty into his counsels, and he accordingly communicated his intention to that Board, with the result that many of the specimens of timber experimented on were sent from Woolwich dockyard, and the results of the trials were from time to time communicated to the Admiralty.
The different blocks of timber under trial were treenailed to the rock, and the experiments extended over a period of nearly thirty years. They clearly proved that teak, African oak, English and American oak, mahogany, beech, ash, elm, and the different varieties of pine, were found sooner or later to become a prey to the Limnoria. Greenheart oak was alone found to withstand their attacks, and even this timber was ultimately not entirely unaffected.
The result of these valuable experiments is given in the following Table:—
157
Table showing the different kinds of Timber which were exposed to the attacks of the Limnoria terebrans at the Bell Rock in 1814, 1821, 1837, 1843, with their durabilities.
Kind of Timber. Decay
first
observed. Unsound
and
quite
decayed. Quite
sound
for Remarks.
yrs. mo. yrs. mo. yrs. mo.
Greenheart,1 19 ?0 1 Affected in one corner.
Teak-wood, 13 ?0
Beef-wood, 13 ?0
Treenail of Bullet-wood, ?5 ?0
Beech, Payne’s patent pro.,2 10 ?7 2 A little holed at one end underneath. Nearly sound 7? years after being laid down.
Teak-wood,3 ?5 ?6
African Oak,4 ?5 ?6 3 Nearly sound 7? years after being laid down.
Do. do. ?4 11 10 ?0 4 Nearly sound 7? years after being laid down.
English Oak, kyanised, ?4 ?7 10 ?0
Teak-wood, ?4 ?7 12 ?0
American Oak, kyanised,5 ?4 ?3 5 Decaying, but slowly, 5 years and 7 months after being laid down.
British Ash, ?3 ?0 ?5 ?0
Scotch Elm, ?3 ?0 ?5 ?0
Ash, ?2 11 ?4 ?3
English Elm, ?2 11 ?4 ?7
Plane Tree,6 ?2 11 6 Decaying, but slowly, 5 years and 7 months after being laid down.
American Oak, ?2 11 ?4 ?7
Baltic Red Pine,7 ?2 ?9 ?4 ?3 7 A good deal decayed when first observed.
English Oak, ?2 ?4 ?4 ?7
Scotch Oak,8 ?2 ?4 8 Much decayed when first observed.
Baltic Oak, ?2 ?4 ?4 ?3
Norway Fir, ?2 ?4 ?3 ?1
Baltic Red Pine, kyanised, ?2 ?4 ?4 ?7
Pitch Pine, ?2 ?4 ?4 ?3
American Yellow Pine, ?2 ?4 ?3 ?7
American Red Pine, ?2 ?4 ?3 ?1
Do. do., kyanised, ?2 ?4 ?4 ?7
Larch, ?2 ?4 ?4 ?3
Honduras Mahogany,9 ?2 ?1 &n............