[Footnote 3: And yet, so strong is the propensity to metaphor, that scientific men talk of the _vis inertiae_ as a true force, though the ideas expressed by the two Latin words are certainly incongruous. The mistake here arises from confounding inertness, or resistance to force,--a merely negative idea,--with the true force which is necessary to overcome it; or rather, since force can only be measured by its results, and must always be adequate to the effect produced, inquirers have adopted the convenient hypothesis of two antagonistic forces, not always recollecting that one of them is merely passive.]