STRENGTH OF MATERIALS. 
from seeds. All require a pretty high tempera- 
ture, and make a very brilliant appearance. 
STRENGTH OF MATERIALS. The force 
with which the particles of a solid body resist 
the forces that tend to separate them, and which 
constitutes their strength, may be found, in par- 
ticular cases, by experiment. If the experiments 
be multiplied, both in respect to the species of 
substances, and to the size and circumstances 
under which portions of a given substance are 
employed, a general law might be finally obtained 
whence algebraic expressions of the relation be- 
tween the strength, the dimensions, and the ma- 
terial, might be deduced. Or we may, by as- 
suming an hypothesis to represent the probable 
manner in which bodies are made up, deduce 
from the general formule, which, applied to 
cases that occur in practice, will be sufficient, 
in almost every instance, to represent the phe- 
nomena. 
Galileo was the author of the hypothesis 
that is most generally employed by writers on 
mechanics, and which will suffice in all usual 
cases. He assumes that all solid bodies are com- 
posed of a great number of parallel and equal 
fibres, perfectly inflexible and inextensible; that 
when they break, the several fibres give way in 
succession, and the body turns upon the fibre or 
fibres that are the last to give way, as upon a 
hinge. Leibnitz observing the flexure that takes 
place in bodies before they break, assumed as 
the basis of his hypothesis, the fact, that every 
body admits, before it breaks, of a certain degree 
of extension: the fibres, therefore, are both ex- 
tensible and flexible; and he inferred that the 
strength of each fibre, instead of being equal, 
varied with its quantity of extension, or was 
proportioned to its distance from the fixed point 
around which the beam is supposed to turn. 
The hypothesis of Galileo has been recently ex- 
tended by Barlow, who has, by introducing the 
circumstance of the occurrence of a flexure be- 
fore the beam breaks, been enabled to explain all 
the cases which appeared, by a comparison of the 
hypothesis with experiment, to be anomalous. 
The force which tends to destroy the aggrega- 
tion of the particles of a solid body, may act in 
four different manners. 
(1.) It may tend to tear the body asunder by 
exerting an action in the direction of its fibres. 
(2.) It may tend to break the body across, by 
a transverse strain, exerted either perpendicu- 
larly or obliquely to the direction of its fibres. 
(3.) It may tend to crush the body. 
(4.) It may tend to separate the particles by 
means of torsion, twisting or wrenching the body, 
by an action in a plane perpendicular to its 
axis. 
The resistance which iborities oppose to a force 
acting in the first of these modes, is called the 
Absolute Strength. In fibrous bodies, it is dif- 
ferent, according as the force is applied in the 
direction of the fibre, or at right angles to it: 
J09 
for although by hypothesis we should conceive 
fibres to exist, even in the transverse direction, 
yet, in nature, none such are found; and the 
aggregation is far more easily destroyed in the 
latter case than in the former. The resistance 
to a fracture across the body, is called its Respec- 
tive Strength ; and we may call the resistance to 
a twisting force, the Strength of Torsion. 
It needs no reasoning to show that the mea- 
sure of the strength, even in the same body, will 
be different in each of these different cases; and 
such is the difference in the manner in which 
the particles of bodies of different natures are 
united, that there can be no general Jaw that 
will represent the relations of these four species 
of resistance. But in conformity with our hypo- 
thesis, it will be obvious that, in all the several 
cases, the resistance to fracture will vary with 
the number of fibres, which, in homogeneous 
bodies, will depend upon the area of their sec- 
tions. It must also depend upon the manner in 
which the force acts to break the body. 
I. Of the Absolute Strength of Materials. When 
the strain is exerted in the direction of the fibres, 
the force that tends to break a body will be 
directly opposed to its force of aggregation, and 
the resistance must depend upon the cohesive 
force of each fibre, and upon their number, but 
upon no other circumstance. 
The most valuable experiments that we have 
of this sort, are those of Barlow, upon wood. 
These were made upon cylindrical rods, and the 
strength, deduced for an element of the area of 
a square inch. The results are contained in the 
second of the following tables. The first table 
has been compiled from various other sources. 
TABLE TI. 
Absolute Strength of the Metals. 
Cast Steel, : 140,000 lbs. 
Gold, (according to Morveau, ) 80,000 
Wrought Iron, (Swedish, ) 72,000 
do. (English, ) 56,000 
do. do. inthe form of chains, 48,000 
Bronze, (Gun Metal, ) ‘ 36,000 
Wrought Copper, 33,000 
Cast do. 19,000 
Brass, 17,000 
Tin, 4,700 
Lead, 1,800 
TABLE II. 
Absolute Strength of different kinds of Wood drawn 
in the direction of their fibres. 
Boxwood, 20,000 Ibs. 
Ash, 17,000 
Teak, ; 15,000 
Norway Fir, 12,000 
’ Beech, 11,500 
Canada Oak, 11,400 
Russia Fir, 10,700 
Pitch Pine, 10,400 
English Oak, : , 10,000 
American White Pine, 9,900 
Pear Tree, ; 9,800 
Mahogany, 8,000 
Woh, x : 5,800 
