Remarks on Inertia. 51 



inanimate, — has no directive power of its own, — no will if 

 we may so speak ; and hence, to cause changes in matter, 

 an external force is necessary, and this must be proportioned 

 to the mass to be operated upon. By the application of 

 such a force, we can change the place of any quantity of 

 matter, and the change will be exactly in proportion to the 

 force. Some speak of ; ' vis inertise," — " fuerza de inertia," 

 — " force d'inertie," and " force of inertia ; " but those phra- 

 ses sound to us like a contradiction in terms ; and we do not 

 believe that they are of any service to the student in natural 

 science : On the contrary, we are convinced that they tend 

 to confuse his reasonings, and suggest the idea of power, 

 when that which should be conveyed is the absence of all 

 power. We are happy in not being singular in this opinion : 

 " This property, (inertia) says Dr. Young, is improperly cal- 

 led a force. 1st. Because, were it actually such it must be of 

 some definite quantity in a given body ; and therefore an im- 

 pressed force less than that would not move the body ; 

 whereas any impressed force, however small, whether impul- 

 sive or constant, will move any body however great ! 2nd. 

 It is improper, because it seems to indicate an active force 

 resident in matter."* 



We by no means think this a subject of small moment : 

 We are continually using language in comparing the thoughts 

 which exist in our own minds, as well as in communicating 

 those thoughts to others : and inaccuracy of expression will 

 be very likely to draw after it inaccuracy of thought. We 

 should endeavor to inform ourselves of the extent of our 

 powers, and never attempt to search into those things which 

 are beyond.our ken : And this cannot be accomplished with- 

 out having fixed and definite notions attached to our words. 

 Many students receive the phrase " vis inertias" without ex- 

 amination ; and when they come to translate it, — " the force 

 of inactivity," which is the more classical meaning, is totally 

 incomprehensible when applied to matter ; and " the force 

 of passiveness " is not much plainer. Others are still more 

 unfortunate : They fix the expression "force of inertia" in 

 their memories, and afterwards look upon it as denoting an 

 active power inherent in matter. 



All inquiries respecting the cause of inertia are as utterly 

 fruitless as those intended to discover the cause of gravita- 



* Analysis of the Principles of Natural Philosophy, p. 2S. 



