40 
ON THE PHYSICAL STRUCTURE OF PLANTS. 
slip of the Brazil wood, tried by this test, came out as colourless, light, 
and fragile as a piece of willow. 
In the government dock-yards they have machines for proving the 
strength of timber, in order to estimate its comparative value or fitness 
for particular purposes. Scantling of three or four inches square, and 
three or four feet long, are laid on two immovable props, somewhat 
less distant from each other than the length of the pieces to be tried; 
weights are heaped upon the middle of the piece till it breaks; the 
flexure and number of pounds' weight under which it breaks being 
noted, is the proof of its comparative strength. This, by-the-by, is 
proof of the toughness of the timber, rather than of its durability, 
because it is well known that in many kinds of timber the latter pro¬ 
perty may exist where the former is wanting. Lignum vitse and ebony 
are very lasting, though fragile; bamboo is both very flexible and very 
durable; willow and ash are flexible, but by no means lasting. 
Cleft scantling is always found more durable in the open air than 
that which is sawn ; the reason is, the saw dismembers the grain, while 
the wedge does not. 
Timber, particularly oak, is found of very different quality when 
brought to the artificer; the heart-wood is only valuable. This has 
had time to become as compact as possible, and to be thoroughly 
impregnated with its peculiar tanning principle. The alburnum or 
white wood of oak is the least durable of any ligneous production, and 
therefore is always slabbed off as useless for any permanent purpose. 
Much has been written on the cause of the inequality of oak timber ; 
indeed it has become a question of very great national concern. While 
beams in ancient cathedrals, piles of ancient bridges, and even the 
timbers of stranded ships which have lain buried up for ages, are found 
as sound as ever, some of our lately-built and noble ships of war have 
been condemned, or have required extensive repair, ere they left the 
stocks. 
Many conflicting opinions have been offered as to the cause of this 
misfortune; in general it has been alleged that the oak lately used in 
the king's and merchants’ yards, has been prematurely felled, or used 
before it has been properly seasoned. Others suppose that the old 
English oak (Quercus navalis, robur , or pedunculata) has been partly 
banished by a spurious variety, of similar appearance, but of far less 
value as timber for ship-building ; and, to guard against this unfor¬ 
tunate substitution, it is only necessary to observe, that the “ true 
naval oak is easily distinguishable from the others growing wild in 
Britain, by the acorns being seated on long stalks, and the leaves sub- 
