On the Resistance of Solids. 415 
anes of glass, in the direction of their length, which returned 
to their original length when the weight was removed *. 
6. The extension or compression of homogeneous bodies ‘is 
directly proportional to the force which occasions it ; at least in 
the first degrees of extension or compression. 
Thus, if a wire of any metal be extended 1-10th of an inch 
by a weight of 100 pounds, it would be extended 2-10ths by a 
weight of 200 pounds; 8-10ths by 800 pounds, &c.+ 5 also, 
if a body be compressed ‘1-l0th of an inch by 109 pounds, it 
would be compressed 2-}0ths by 200 pounds, &c. 
7. In the first degrees of extension or compression, all homo- 
geneous bodies are extended and compressed in an equal degree 
by equal forces. That is, if a foree of 1060 pounds would ex- 
tend a body 10th of its patural length, it would be compressed 
1-10th of its nataral length by the same force. 
It is justly observed by a late writer on this subject, that ‘so 
far as this doctrine has been investigated by experiments, its 
general truth has been amply confirmed; the slight deviations 
from the exact proportion, which have been discovered in some 
substances, being far too unimportant to constitute an exception, 
and merely tending ‘to show that these substanees cannot have 
been periectly homogeneous, in the sense here attributed to:the 
word ¢.” In making experiments on the extension and com- 
pression of bodies, the times of action have net often been at- 
tended to with a sufficient degree of accuracy. It is well known 
that a certain time must elapse befere the weight produces its 
full effect upon the body, partieularly when the weight is consi- 
-derable ; and from a few experiments of ny ewn, Iam inclined 
to think, that were the weight suffered to,produce its full.effect 
at each operation, we should find the extension exactly propor- 
tional to the ferce, even to the time of fracture, and it is most 
probable that the same observation will apply to,compression. 
8. When a bar or beam is fixed at one end, ina herizontal 
position, and a sufficient weight suspended at the other end, the 
bar will break at the point of support; and the following phe- 
nomena will take place. 
9. The bar will bend, and the bending will be proportional to 
the extensibility of the material. 
10. The upper side of the bar will be extended, and the lower 
* Traité du Mouvement des Eaux, Sect. v. Discours ii. 
+ Some experiments of this kind weré made by Emerson,—Mechanics, 
Prop. 76. Ed. 1773. Bernouilli has attempted to demorstrate that the 
- compression cannot be proportional to the force ; but his reasoning applies 
to extreme cases only; and the result of the solitary experiment he made 
is completely different from those of every other writer that I have consulted. 
See his paper in the Mem. de 0 Acad. dea Scien. Paris 1705, p. 179. 
{ Supplement to Encyclop. Britannica, art. Bridge, p. 497. 1817. i 
side 
