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] 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].
[18] Phil. Trans. 1827, p. 25.
[19] 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.