362 STRENGTH OF MATERIALS. 
III. Of the Resistance of Bodies to a Force ex- 
erted to crush them.—The resistance of bodies to 
forces that act to crush them, would, at first 
sight, appear to follow the same law with the 
absolute strength; that is to say,—it must be 
proportioned to the area of the body, and the 
force of aggregation of its particles. Experi- 
ment, however, shows, that the thickness of the 
substance has an important influence on the 
pressure it is capable of bearing, without being 
crushed. In the first place, it is found that 
very thin plates are readily crushed; their re- 
sistance next increases with the thickness, until 
it reach a maximum; and finally decreases 
slowly, with a farther increase of thickness. 
This branch of the subject has been illustrated 
under the head PRESSURE. 
In bodies whose sections are similar, it may be 
inferred that the resistance to a force exerted to 
crush them, is proportioned directly to the cubes 
of the homologous dimensions of the sections, 
and inversely to the squares of their lengths. 
The most complete set of experiments that we 
have upon the variation in the strength of 
columns of different lengths, are those of Mus- 
chenbrook, and they correspond closely with the 
above theory. The subject, however, has not 
been so fully tested as to authorize us to receive 
any theory with implicit confidence. We shall, 
in consequence, give the absolute results of ex- 
periments, in the terms of the weights and dimen- 
sions of the original record, without attempting 
to reduce them to a conventional unit. 
TABLE. 
Experiments made by Rondelet, on the Resistance of 
different species of Stone in cubic blocks, of the 
size of 5 centimetres in each dimension. 
Spec. Crushing Weight. 
Gravity. Kilogrammes, 
Swedish Basalt, “ 3:065 47 809 
Basalt of Auvergne, No. 1, 3°014 44:250 
Do. No. 2, 2°884 51:945 
Do. No. 3, 2°756 28°858 
Porphyry, 2°798 50-021 
Green Granite (Vosges), No. 1, 2°854 15-487 
Grey do. Sa 2°737 =16°353 
Granite (Vosges), No. 2 2°664 20-482 
Granite (Normandy), 2°662 17°555 
Granite (Champ du Boul), 2°643 20°441 
Granite (Oriental Rose), 2°662 22-004 
Black Marble (Flanders), . 2721 ' 19-719 
White Veined Marble, 2-701 7°455 
White Statuary Marble, 2°695 8:°176 
Experiments made at the same time upon 
cubes of stone, whose sides varied from 3 to 6 
centimetres, showed that the strengths varied 
nearly in proportion to the areas of their bases, 
and were not influenced by the thickness. This 
corresponds to the law of absolute strength, and 
differs from that we have given for the resistance 
to crushing. On the other hand, when cubes of 
the same size were placed one upon another, a 
diminution in the resistance was found that 
does not differ much from the latter law. 
By experiments made by Rennie, on the re- 
STREPTANTHUS. 
sistance of cast iron to pressure, it was found 
that the maximum strength was reached at 
thicknesses of from 2ths to 4 an inch, and that 
a prism a quarter of an inch square, and half an 
inch in depth, was crushed by 10,000 lbs. 
A cube of cast copper, 4 
crushed by 7,318 lbs. 
A cube of wrought copper, of the same size, by 
6,440 lbs. 
Of brass, by 10,304 lbs. 
Of cast tin, by 966 lbs. 
Of lead, by 483 lbs. 
An inch cube of elm is crushed by 1,284 Ibs. 
Of American pine, by : 1,606 
Of Norway fir, by 1,928 
Of English oak, by 3,860 
IV. Of the Strength of Torsion—By the ex- 
periments of Coulomb, the resistance of wires of 
the same material, to a force exerted to twist 
them, appears to increase with the fourth power 
of their diameters. A similar result follows 
nearly, from experiments by Rennie, on square 
bars of cast iron. It would also appear, from a 
simple course of reasoning, that the resistance 
must diminish with the distance of the point in 
the rod to which the twisting force is applied, 
from the place where it is fixed; for the rod 
tends, under the action of the force, to form a 
screw, the distance between whose threads is 
the same as the distance between the two points. 
And in the inclined plane which the screw forms 
when developed, the twisting force will act as if 
it tended to raise a weight along the surface of 
the plane, whose altitude is the constant lineal 
dimension of the base of the rod. 
We have no experiments on the absolute re- 
sistance to torsion ; the following are the relative 
resistances of different materials, deduced from 
the experiments of Rennie. 
inch each way, was 
Lead, 1,000 
Tin, 1,438 
Copper, 4,312 
Brass, 4,688 
Gun Metal, 5,060 
Swedish Iron, 9,500 
English Iron, 10,125 
Cast Iron, 10,660 
Blister Steel, 16,688 
Sheer Steel, 17,063 
Cast Steel, 19,562 
We are indebted for the abies “oe Ral to Dr. 
Renwick’s Elements of Mechanics. 
STREPTANTHERA. A genus of ornamental, 
bulbous-rooted, exotic plants, of the iris order. 
It takes its name from the twistedness of its an- 
thers. The elegant species, S. elegans, is the old- 
est known one, and was introduced to Britain 
about 22 years ago from the Cape of Good Hope. 
It has a height of about 5 or 6 inches; and car- 
ries flowers of white and blue and yellow in May 
and June; and loves a soil of mixed sand and 
peat and loam. 
STREPTANTHUS. A recently discovered 
genus of ornamental exotic plants, of the cruci- 
ferous order. The blunt-leaved and the hya- 
cinth-flowered species are hardy, late-blooming 
