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 9 nearly sound 3? years after being laid down. washed away 6 months later.

beech, ?1 ?9 ?3 ?1

american elm, ?1 ?9 ?3 ?1

treenail of locust, ?5 ?0 ?3 ?0

british oak, ?1 ?6 ?5 ?0

american oak, ?1 ?6 ?5 ?0

plane tree, ?1 ?6 ?5 ?0

honduras teak treenails, ?1 ?6 ?5 ?0

beech, ?1 ?6 ?5 ?0

scotch fir, teak treenails, ?1 ?6 ?3 ?0

do. from lanarkshire, ?1 ?6 ?3 ?0

do. do. ?1 ?6 ?3 ?0

do. locust treenails, ?1 ?6 ?3 ?0

memel fir, ?1 ?6 ?5 ?0

pitch pine,10 ?1 ?6 ?2 ?6 10 going fast when first observed.

english oak, ?1 ?1 ?3 ?1

italian oak, ?1 ?1 ?3 ?6

dantzic oak, ?1 ?1 ?2 ?6

english elm, ?1 ?1 ?1 ?6

canada rock elm, ?1 ?1 ?1 ?6

cedar of lebanon, ?1 ?1 ?2 ?6

riga fir, ?1 ?1 ?1 ?6

dantzic fir, ?1 ?1 ?1 ?6

virginia pine, ?1 ?1 ?1 ?6

yellow pine,11 ?1 ?1 ?1 ?6 11 a good deal gone 18 months after being laid down. swept away by the sea 7 months afterwards.

red pine, ?1 ?1 ?1 ?6

cawdie pine,12 ?1 ?1 ?1 ?6 12 a good deal decayed when first observed.

polish larch,13 ?1 ?1 ?1 ?6 13 going fast when first observed.

birch, payne’s patent pro., ?0 10 ?1 10

american locust treenails, ?0 ?8 ?3 ?0

158 mr. stevenson seems to have formed an opinion that the best preservative against decay was charring the timber, as recommended in the following extract from a report, made in 1811, to the trustees of montrose bridge:—

“the changeableness of climate to which the northern parts of this island are subject renders edifices of timber more liable to decay here than perhaps in any other country in europe. but the bridge at montrose is curiously circumstanced; for while it unavoidably exposes a great surface of timber to the action of the weather, some of the wooden piers are immersed twenty-two feet in the water, where they are attacked by a destructive marine worm. some of the woodwork at the bell rock was infested with the same species of animal which preys upon the wooden pier at montrose. in some of the temporary works there, as in the beams laid for carrying the railway over the inequalities of the rock, the timber was so much wormed that some logs measuring one foot when laid down would not square to more than nine inches at the end of three years. the beams which supported the wooden house for the accommodation of the artificers while the lighthouse was erecting escaped almost untouched, having been slightly charred, but the reporter, when inspecting the bell rock works this year, found that these worms are making some impression upon the ends of the supports resting on the rock where the charring could not take effect. the reporter is therefore of opinion that there is no better defence against the effects of this animal than slightly charring the timber, and he would recommend the practice at the bridge of montrose wherever it can be applied. the operation of charring at the bell rock was performed by previously scraping off the adhering matter upon the logs and laying the skin of the wood open, and tar was applied to promote the combustion. charcoal, besides being tasteless and inodorous,159 possesses some very curious properties in its action upon vegetable and animal substances, which may not only render it insipid, but even offensive to this insect. for those parts between the high-water mark and the roadway it will be enough to scrape the timber and lay it over with hot tar.”

i need hardly say that this advice would perhaps not have been given at the present day, when even creosote has been found to delay, though not to act as a perfect defence against, the ravages of the limnoria.10

preservation of iron.

at a more recent period mr. stevenson experimented at the bell rock lighthouse in the same way on twenty-five different kinds of malleable iron, with the result that all of them were soon affected, and that galvanised specimens resisted oxidation from three to four years, after which the chemical action went on as quickly as in the others.