We have had before us a sufficient number of examples of scientific terms thus framed; for they formed the first of three classes which we described in the First Aphorism. And we may again remark, that science, when she thus adopts terms which are in common use, always limits and fixes their meaning in a technical manner. We may also repeat here the warning already given respecting terms of this kind, that they are peculiarly liable to mislead readers who {279} do not take care to understand them in their technical instead of their common signification. _Force_, _momentum_, _inertia_, _impetus_, _vis viva_, are terms which are very useful, if we rigorously bear in mind the import which belongs to each of them in the best treatises on Mechanics; but if the reader content himself with conjecturing their meaning from the context, his knowledge will be confused and worthless. In the application of this Third Aphorism, other rules are to be attended to, which I add. APHORISM IV. _When common words are appropriated as technical terms, their meaning and relations in common use should be retained as far as can conveniently be done._ I WILL state an example in which this rule seems to be applicable. Mr Davies Gilbert[18\4] has recently proposed the term _efficiency_ to designate the work which a machine, according to the force exerted upon it, is capable of doing; the work being measured by the weight raised, and the space through which it is raised, jointly. The usual term employed among engineers for the work which a machine actually does, measured in the way just stated, is _duty_. But as there appears to be a little incongruity in calling that work _efficiency_ which the machine _ought_ to do, when we call that work _duty_ which it really does, I have proposed to term these two quantities _theoretical efficiency_ and _practical efficiency_, or _theoretical duty_ and _practical duty_[19\4]. [Note 18\4: _Phil. Trans._ 1827, p. 25.] [Note 19\4: The term _travail_ is used by French engineers, to express _efficiency_ or _theoretical duty_. This term has been rendered in English by _labouring force_.] Since common words are often vague in their meaning, I add as a necessary accompaniment to the Third Aphorism the following:-- {280} APHORISM V. _When common words are appropriated as technical terms, their meaning may be modified, and must be rigorously fixed._ THIS is stated by Bacon in the above extract: 'to retain the ancient terms, though I sometimes _alter the uses and definitions_.' The scientific use of the term is in all cases much more precise than the common use. The loose notions of _velocity_ and _force_ for instance, which are sufficient for the usual purposes of language, require to be fixed by exact measures when these are made terms in the science of Mechanics.