This class of technical terms, namely, words adopted from common language, but rendered precise and determinate for purposes of science, may also be exemplified in other sciences. Thus, as was observed in the early portion of the history of astronomy[1\4], a _day_, a _month_, a _year_, described at first portions of time marked by familiar changes, but afterwards portions determined by rigorous mathematical definitions. The conception of the heavens as a revolving sphere, is so obvious, {260} that we may consider the terms which involve this conception as parts of common language; as the _pole_ (πόλος); the _arctic circle_, which includes the stars that never set[2\4]; the _horizon_ (ὁρίζων) a boundary, applied technically to the circle bounding the visible earth and sky. The _turnings of the sun_ (τροπαὶ ἠελίοιο), which are mentioned by Hesiod, gave occasion to the term _tropics_, the circles at which the sun in his annual motion turns back from his northward or southward advance. The _zones_ of the earth, (the _torrid_, _temperate_, and _frigid_;) the _gnomon_ of a dial; the _limb_ (or border) of the moon, or of a circular instrument, are terms of the same class. An _eclipse_ (ἔκλειψις) is originally a deficiency or disappearance, and joined with the name of the luminary, an _eclipse of the sun_ or _of the moon_, described the phenomenon; but when the term became technical, it sufficed, without addition, to designate the phenomenon. [Note 1\4: _Hist. Ind. Sci._ b. iii. c. i.] [Note 2\4: _Hist. Ast._ b. iii. c. i. sect. 8.] In Mechanics, the Greeks gave a scientific precision to very few words: we may mention _weights_ (βάρεα), the _arms of a lever_ (μήχεα), its _fulcrum_ (ὑπομόχλιον), and the verb _to balance_ (ἰσσοῤῥοπεῖν). Other terms which they used, as _momentum_ (ῥοπὴ) and _force_ (δύναμις), did not acquire a distinct and definite meaning till the time of Galileo, or later. We may observe that all abstract terms, though in their scientific application expressing mere conceptions, were probably at first derived from some word describing external objects. Thus the Latin word for force, _vis_, seems to be connected with a Greek word, ἲς, or ϝὶς, which often has nearly the same meaning; but originally, as it would seem, signified a sinew or muscle, the obvious seat of animal strength.