Principles of Energetics. 275 
Mechanical, as distinguished from Biological Forces. And 
without such a theory it is evident that no general and concur- 
rent laws or relations can be established between phenomena of 
Motion, as distinguished from phenomena of Growth. Attrac- 
tions (gravic, electric, and magnetic) and Waves (acoustic, optic, 
and thermotic) are the motions offered by Physics for explana- 
tion by mechanical forces, or conditions of pressure. The con- 
stitution and combination of bodies are the phenomena of which 
Chemics require a similar mechanical interpretation. These 
sciences may be distinguished from Mechanics, with its ordinary 
limitation of meaning, as forming together the science of Mole- 
cular Mechanics. But in Ordinary Mechanies also there are 
phenomena, the causal relations of which have been hitherto as 
little established as those of the phenomena of Attraction or of 
Affinity. Such unexplained mechanical phenomena are the uni- 
form motion of the planets, and their velocities of rotation, as yet 
unconnected even by an empirical law. 
The principles of Energetics more particularly belonging to 
Ordinary Mechanics will therefore be in this paper applied to 
the explanation of these mechanical facts. 
4, (I.) A Force is the condition of difference between two pres- 
sures in relation to a third. 
5. To establish the principles of Energetics applicable to the 
first part of Mechanics, it seems unnecessary to use the term pres- 
surewith other than its usual limited meaning as Statical pressure. 
Such a meaning would at least be wide enough for this first prin- 
ciple. For it might be otherwise expressed :—the general cause of 
the movement of a body is a difference between two (previously) 
equilibratmg pressures upon it. But in order that it may be 
seen, at least generally, how I propose to bring the idea of Pres- 
sure into Physics and Chemies, and hence to make this principle 
the foundation of Molecular as well as of Ordinary Mechanics, 
it may be well to state at once that “ under the term Pressure I 
shall include every kind of force which acts between elastic 
- bodies, or the parts of an elastic body, as the cause (condition) 
or effect of a state of strain, whether that force is tensile, com- 
pressive, or distorting* ;” and that I consider elasticity to be 
“une des propriétés générales de la matiére. lle est en effet 
Porigine réelle, ou ’intermédiaire indispensable des phénoménes 
physiques les plus importans de l’univers..... La gravitation et 
Pelasticité doivent étre considérées comme les effets d’une méme 
cause qui rend dépendantes ou solidaires toutes les parties maté- 
rielles de l’univers +.” 
* Rankine, Camb. and Dub. Math. Journ. 1851, vol. 11. p. 49. 
+ Lamé, Théorie Mathématique de ?’ Elasticité des Corps Solides, pp. 1 
2 
and 2, 
T2 
