INTRODUCTION. 13 



qualities of all bodies existing :" fourthly, " in experimental philosophy, 

 propositions collected by induction from phenomena, are to be esteemed 

 either accurately or very nearly true, notwithstanding any contrary hypo- 

 thesis, until other phenomena occur by which they may either be corrected 

 or confuted." 



As an illustration of the remark, that these axioms, though strictly true, 

 are of little real utility in assisting our investigations, I shall give an in- 

 stance from the subject of electricity. Supposing that we wish to determine 

 whether or no the electric fluid has weight ; we are to inquire whether or no 

 gravitation is one of those properties which are described in the third rule, 

 and whether that rule will authorise us to apply it to the electric fluid, as 

 one of those qualities of bodies which cannot be increased nor diminished, 

 which are found in all bodies within the reach of our experiments, and 

 which are, therefore, to be considered as general qualities of all bodies 

 existing. Now it appears to be, in the first place, uncertain whether or no 

 the increase and diminution of gravity, from a change of distance, is strictly 

 compatible with the terms of the definition ; and, in the second place, we 

 are equally at a loss to decide, whether or no the electric fluid can with 

 propriety be called a body ; for it appears in some respects to be wholly 

 different from tangible matter, while it has other qualities in common with 

 it. Such are the difficulties of laying down general laws on so comprehen- 

 sive a scale, that we shall find it more secure to be contented to proceed 

 gradually by closer inductions in particular cases. We shall, however, 

 seldom be much embarrassed in the choice of a mode of argumentation. The 

 laws of motion, which will be the first immediate subjects of discussion, 

 have indeed sometimes been referred to experimental evidence ; but we 

 shall be able to deduce them all in a satisfactory manner, by means of our 

 general axiom, from reasonings purely mathematical, which, wherever they 

 are applicable, are unquestionably preferable to the imperfect evidence of 

 the senses, employed in experimental investigations.* 



LECTURE II. 



ON MOTION. 



THE whole science of mechanics depends on the laws of motion, either 

 actually existing or suppressed by the opposition of the forces which tend 

 to produce it. The nature of motion requires, therefore, to be particularly 

 examined at the entrance of the science of mechanical philosophy ; and 

 although the subject is so abstract as to demand some effort of the attention, 

 being seldom capable of receiving much immediate illustration from the 

 objects of sense, yet we shall find it indispensable to our progress in the 



* Consult Stewart's Philosophy of the Human Mind, 2 vols. 1818-21, v. 2. Brown 

 on Cause and Effect. Whewell's Philosophy of the Inductive Sciences, 2 vols. 

 1840. 



