Dynamical Units

Abstract

IF Prof. Perry's reply to my letter (on p. 126) is summed up in the charge that I think of “stuff” when I ought to be thinking of inertia, then the issue between us should reduce to very minute dimensions. It is, perhaps, unfortunate that in English the term mass may signify either “quantity of matter” or the inertia of that matter, but hardly so unfortunate as the fact that weight may denote either a quantity of matter or a force, an ambiguity to which we are all prone, though Prof. Perry makes light of it; for a definite amount of matter implies, at least, a definite amount of inertia, but not a definite weight, in the sense of force If Prof. Perry thinks that with himself the word mass means simply inertia, then, substituting at the bottom of p. 49 in the current volume, I find that he says: “My unit of inertia is the inertia [the italics are mine] to which unit force gives an acceleration of 1 foot per second per second.” I am free to confess that I cannot dissociate the conception of inertia from the idea of matter; but here we have abstraction indeed.