Mr. AV. J. Macquorn Rankine on the Science of Energetics. 383 



tive hypotheses may be taken, that of vibrations or oscillations in the 

 theory of light, and that of atoms in chemistry ; as an example of a sub - 

 jective hypothesis, that of magnetic fluids. 



III. The Science or Mechanics considered as an illustration of 

 THE Abstractive Method. 



The principles of the science of mechanics, the only example yet exist- 

 ing of a complete physical theory, are altogether formed from the data of 

 experience by the abstractive method. The class of objects to which the 

 science of mechanics relates, — viz., — material bodies, — are defined by 

 means of those sensible properties which they all possess, viz., the pro- 

 perty of occupying space, and that of resisting change of motion. The 

 two classes of phenomena to which the science of mechanics relates are 

 distinguished by two words, motion and force ; motion being a word de- 

 noting that which is common to the fall of heavy bodies, the flow of 

 streams, the tides, the winds, the vibrations of sonorous bodies, the revo- 

 lutions of the stars, and generally to all phenomena involving change of 

 the portions of space occupied by bodies ; and force, a word denoting that 

 which is common to the mutual attractions and repulsions of bodies, dis- 

 tant or near, and of the parts of bodies, the mutual pressure or stress of 

 bodies in contact, and of the parts of bodies, the muscular exertions of 

 animals, and, generally, to all phenomena tending to produce or to pre- 

 vent motion. 



The laws of the composition and resolution of motions, and of the com- 

 position and resolution of forces, are expressed by propositions which are 

 the consequences of the definitions of motion and force respectively. The 

 laws of the relations between motion and force are the consequences of 

 certain axioms, being the most simple and general expressions for all that 

 Las been ascertained by experience respecting those relations. 



IV. Mechanical Hypotheses in various Branches of Physics. 



The fact that the theory of motions and motive forces is the only com- 

 plete physical theory, has naturally led to the adoption of mechanical 

 hypotheses in the theories of other branches of physics ; that is to say, 

 hypothetical definitions, in which classes of phenomena are defined con- 

 jecturally as being constituted by some kind of motion or motive force 

 not obvious to the senses (called molecular motion or force) as when light 

 and radiant heat as defined as consisting in molecular vibrations, thcrmo- 

 metric heat in molecular vortices, and the rigidity of solids in molecular 

 attractions and repulsions. 



The hypothetical motions and forces are sometimes ascribed to hypo- 



