TIMBER. 
square 1)3' the decimal 0.079577, and the 
product by the lengith. Example. Let the 
meat) cii cnmference of a tree be 10.3 feet, 
and the length 24 feet. Then 10.3 X 10.3 
X 0.07 9577X24= 202.615, the number of 
cubical feet in the tree. The founda- 
tion of this rule is, that when the cir- 
cumference of a circle is 1 , the area is 
0.0795774715, and that the areas of circles 
are as the squares of their circumferences. 
But the common way used by artificers for 
measuring round timber, ditFers much from 
this rule. They call one fourth part of the 
circumference the girth, which is by them 
reckoned the side of a squai e, whose area 
is equal to the area of the section of the 
tree} therefore they square the girth, and 
then multiply by the length of the tree. 
According to their method, the tree of the 
last example would be computed at 159.13 
cubical feet only, y 
In speaking of the strength of timber, of 
of several kinds of wood, Mr. Emerson 
says, that Irom experiments which he has 
made, a piece of good oak, an inch square 
and a yard long, supported at both ends, 
will bear in the middle for a short time 
about 3301b*. avoirdupoise, but will imme- 
diately break with a greater weight. Such 
a piece, he adds, ought not in practice to 
be trusted for any length of time with more 
than one-third, or perhaps one-fourth .part 
of the weight ; he then gives a table of the 
different degrees of strength of several sorts 
of wood. Other writers who have entered 
at large on the subject, have considered the 
strength of materials, timber, &c. as subject 
to four different kinds of strain. 1. As 
they may be torn asunder, as in the case of 
ropes, stretchers, king-posts, tye-beams, 
&c. 2. As they may be crushed, as in the 
case of pillars, posts, and truss-beams. 3. 
As they may be broken across, as happens 
. to a joist or lever of any kind. 4. As they 
may be wrenched or twisted, as in the case 
of the axle of a wheel, the nail of a press, 
&c. It would carry us much beyond the 
limits ofU)is work to enter at large on these 
several subjects, we shall therefore confine 
ourselves to some observations on the 
•strains upon timber, which may be practi- 
cally useful. 
With regard to the cohesion of wood we 
may premise, 1 . that the wood immedi- 
ately surrounding the pith, or heait, of the 
tree is the weakest, and its infei-iority is so 
much more remarkable as the tree is older. 
This at least is asserted by Muschenbroek 
as tile result of experiments^ but M. Buflon 
says, that his experience has taught him that 
the heart of a sound tree is the strongest ; 
but he gives no instances. It is certain, 
from many observations on very large oaks 
and firs, that the heart is much weaker 
than the exterior parts. 2 . The wood next 
the bark, commonly called the white or 
blea, is also weaker than the rest ; and the 
wood gradually increases in strength as we 
recede from the centre to the blea. 3. The 
wood is stronger in the middle of the trunk 
than at the springing of the branches oi' at 
the root } and the wood ot the branches is 
weaker than that of the trunk. 4. The 
wood of the north side of all trees which 
grow in our European climates is the weak- 
est, and that of the south-east side is the 
strongest } and the difference is most re- 
markable in hedge row trees, and such as 
grow singly. The heart of a tree is never 
in its centre, but always nearer to the north 
side, and the annual coats of wood are 
thinner on that side. In conformity with 
this, it is a general opinion of carpenters, that 
timber is stronger whose annual plates are 
thicker. The trachea, or air-vessels, are 
weaker than the simple ligneous fibres. 
These air-vessels are the same in diameter 
and number of rows in trees of the same 
species, and they make the visible separa- 
tion between the annual plates. Therefore 
when these are thicker, they contain a 
greater proportion of the simple ligneous 
fibres. 5. All woods are more tenacious 
while green, and lose very considerably by 
drying after the trees are felled. The only 
author who has put it in our power to judge 
of the propriety of his experiments is 
Muschenbroek. He has described his me- 
thod of trial minutely, and it seems unex- 
ceptionable. The woods were all formed 
into slips fit for his apparatus, and part of 
the slip was cut away to a parallelopiped of 
one-fifth of an inch square, and therefore 
one-twenty-fifth of a square inch in section. 
The absolute strengths of a square inch 
were as follow; 
lbs. 
Locust tree 
Jiijeb 
Beech, oak 
Orange 
Alder 
Elm 
Mulberry 
Willow 
Ash 
Plum 
Elder 
