1. _Force._--WHEN the faculties of observation and thought are developed in man, the idea of causation is applied to those changes which we see and feel in the state of rest and motion of bodies around us. And when our abstract conceptions are thus formed and named, we adopt the term _Force_, and use it to denote that property which is the cause of motion produced, changed, or prevented. This conception is, it would seem, mainly and primarily suggested by our consciousness of the exertions by which we put bodies in motion. The Latin and Greek words for _Force_, Vis, Ϝὶς, were probably, like all abstract terms, derived at first from some sensible object. The original meaning of the Greek word was a _muscle_ or _tendon_. Its first application as an abstract term is accordingly to muscular force: Δεύτερος αὖτ' Αἴας πολὺ μείζονα λᾶαν ἀείρας ἦκ' ἐπιδινήσας, ἐπέρεισε δὲ ϜÎ͂Ν' ἀπέλεθρον. Then Ajax a far heavier stone upheaved, He whirled it, and impressing Force intense Upon the mass, dismist it. The property by which bodies affect each other's motions, was naturally likened to that energy which we exert upon them with similar effect: and thus the labouring horse, the rushing torrent, the descending weight, the elastic bow, were said to exert force. {206} Homer[8\3] speaks of the _force_ of the river, Ϝὶς ποταμοῖο; and Hesiod[9\3] of the _force_ of the north wind, Ϝὶς ἀνέμου βορέαο. [Note 8\3: _Il._ xxi.] [Note 9\3: _Op. et D._] Thus man's general notion of force was probably first suggested by his muscular exertions, that is, by an act depending upon that muscular sense, to which, as we have already seen, the perception of space is mainly due. And this being the case, it will be easily understood that the _Direction_ of the force thus exerted is perceived by the muscular sense, at the same time that the force itself is perceived; and that the direction of any other force is understood by comparison with force which man must exert to produce the same effect, in the same manner as force itself is so understood. This abstract notion of Force long remained in a very vague and obscure condition, as may be seen by referring to the History for the failures of attempts at a science of force and motion, made by the ancients and their commentators in the middle ages. By degrees, in modern times, we see the scientific faculty revive. The conception of Force becomes so far distinct and precise that it can be reasoned upon in a consistent manner, with demonstrated consequences; and a genuine science of Mechanics comes into existence. The foundations of this science are to be found in the Axioms concerning causation which we have already stated; these axioms being interpreted and fixed in their application by a constant reference to observed facts, as we shall show. But we must, in the first place, consider further those primary processes of observation by which we acquire the first materials of thought on such subjects.