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257 
above the critical point and collapsing when degrudation ensues. 
The destructive effects are on the dynamical theory, explained 
as stresses of force at work internally, produced by strain. The 
ease of strain in the example supposed, that of granite, is 
particular. It is known as shearing stress, which is dynamically 
a tangential force tending to separate a body by making its 
parts slide one upon the other in opposite directions to resist 
shearing motion, and for this reason it is very often called 
shearing stress; this is the physical point of view. Writers on 
the mathematical theory of perfectly elastic solids, often adopt 
the same view as engineers. This may be described as looking 
at all the phenomena of strain from an outside point of view. 
Each body or portion of a body is regarded not as an agent 
opposing strain of its own substance by the exertion of stress, 
but as passively yielding to the stresses exerted on it from 
without. This is the mechanical view of such cases. Energy 
acting thus, actually does work in the moving of matter, 
overcoming rigidity of the mass, and causing distortion and 
displacement of the body. The publications upon the Mathe- 
matical Theory of Elasticities, embracing and treating on the ~ 
properties of strain and stress, are already so many, that in the 
words of a mathematical author, they constitute “the enormous 
literature of Elasticity.” - 
APPENDIX 
Designation of grades of High Temperature. 
The information placed under the above heading, will, it is 
thought, be of some service to an intelligent inquirer in aiding 
him to form a conception of what is understood by degrees of 
high or elevated temperature. Such degrees are mentioned in 
the annexed lists, and without help of the kind, the citations 
would be vague and unmeaning. The subject is attended with 
difficulty, and few indications of this nature are printed ; there 
are notes extant, but in general only to be found incidentally in 
the Transactions of Scientific Societies. By far the best, as far as 
they go, are those fixed by M. M. Fouqué & Lévy, the French 
chemists, who achieved such brilliant results in their experiments 
