686 
MR. fairbairn’s experlmental inquiry 
higher specific gravity, is better calculated to resist shocks than a tough fibrous 
substance of a soft and spongy nature, such as fir. 
On this subject it should however be noticed, that whatever material is used for 
covering the ribs of vessels, it should be strong and elastic, in order to resist not 
only the force of direct tension, but that of lateral and compressed action. In a ship 
at sea these forces are strikingly exemplified, and that under circumstances embar- 
rassing as well to the practical builder as the man of science. 
Remarhs on the foregoing experiments. 
Having determined the strength of iron plates when drawn in the direction of the 
fibre as well as across it, and having compared the results with experiments of a 
similar character on timber, it may be useful to offer a few general observations on 
the question now under consideration. 
Dr. Robinson, in his article on the strength of materials*, when discussing the na- 
ture of a stretching force applied to materials, observes, that in pulling a body 
asunder the force of cohesion is directly opposed with very little modification of its 
action ; that all parts are equally stretched, and the strain in every transverse sec- 
tion is the same in every part of that section.” From this it would appear, that a 
body of a homogeneous texture will have the cohesion of its parts equal, and since 
every part is equally stretched, it follows that the particles will be drawn to equal 
distances, and the forces thus exerted must be equal. Now if this were true, the ap- 
plication of an external force to a body might be increased to such an extent as not 
only to separate the parts furthest asundei*, but ultimately to destroy the cohesion of 
all the particles at once, a circumstance under which instantaneous rupture would 
follow as a result. These views are however not borne out by facts, as the experi- 
ments of Mr. Hodgkinson on iron wire show that the same iron may be torn asun- 
der many times in succession without impairing its strength'!'; and some recent expe- 
riments at the Royal Dockyard, Woolwich, clearly show, that an iron bar may be 
stretched until its transverse section is considerably reduced and ultimately broken 
without injury to its tensile strength. Nay, more, the same iron (so elongated), when 
again submitted to experiment, exhibited increased strength, and continued to in- 
crease, under certain limitations, beyond the bearing powers of the same bar in its 
original forra;|:. That all the parts of a body subjected to a tensile strain are equally 
stretched ” is therefore questionable. Bodies vary considerably in their powers of re- 
sistance, and exhibit peculiar properties of cohesion under the influence of forces cal- 
culated to tear them asunder. Fibrous substances, for instance, such as ropes and 
some kinds of timber having their fibres twisted, are enabled to resist tension under 
the influence of considerable elongation without impairing their ultimate strength. 
* Encyclopedia Britannica. f Manchester Memoirs, vol. v. 
J I am indebted to Mr. Thomas Loyd of the Admiralty for a series of interesting results on this subject. 
See Appendix. 
