The Maxims which belong to the construction of such words will be stated hereafter; but I may notice an instance or two tending to show the necessity of the Maxim now before us. The word _Force_ has been appropriated in the science of Mechanics in two senses: as indicating the cause of motion; and again, as expressing certain measures of the effects of this cause, in the phrases _accelerating force_ and _moving force_. Hence we might have occasion to speak of the accelerating or moving force _of_ a certain _force_; for instance, if we were to say that the force which governs the motions of the planets resides in the sun; and that the accelerating force _of_ this _force_ varies only with the distance, but its moving force varies as the product of the mass of the sun and the planet. This is a harsh and incongruous mode of expression; and might have been avoided, if, instead of _accelerating force_ and _moving force_, single abstract terms had been introduced by Newton: if, for instance, he had said that the velocity generated in a second measures the _accelerativity_ of the force which produces it, and the momentum produced in a second measures the _motivity_ of the force. The science which treats of heat has hitherto had no special designation: treatises upon it have generally been termed treatises _On Heat_. But this practice of employing the same term to denote the property and the science which treats of it, is awkward, and often ambiguous. And it is further attended with this inconvenience, that we have no adjective derived from the name of the science, as we have in other cases, when we speak of _acoustical_ experiments and _optical_ theories. This inconvenience has led various persons to suggest names for the Science of Heat. M. Comte {284} terms it _Thermology_. In the _History of the Sciences_, I have named it _Thermotics_, which appears to me to agree better with the analogy of the names of other corresponding sciences, _Acoustics_ and _Optics_. _Electricity_ is in the same condition as Heat; having only one word to express the property and the science. M. Le Comte proposes _Electrology_: for the same reason as before, I should conceive _Electrics_ more agreeable to analogy. The coincidence of the word with the plural of Electric would not give rise to ambiguity; for _Electrics_, taken as the name of a science, would be singular, like _Optics_ and _Mechanics_. But a term offers itself to express _common_ or _machine Electrics_, which appears worthy of admission, though involving a theoretical view. The received doctrine of the difference between Voltaic and Common Electricity is, that in the former case the fluid must be considered as in motion, in the latter as at rest. The science which treats of the former class of subjects is commonly termed _Electrodynamics_, which obviously suggests the name _Electrostatics_ for the latter.