I. The simplest and first mode of acquiring technical terms, is to take words current in common usage, and by rigorously defining or otherwise fixing their meaning, to fit them for the expression of scientific truths. In this manner almost all the fundamental technical terms of Geometry were formed. A _sphere_, a _cone_, a _cylinder_, had among the Greeks, at first, {259} meanings less precise than those which geometers gave to these words, and besides the mere designation of form, implied some use or application. A _sphere_ (σφαῖρα) was a hand-ball used in games; a _cone_ (κῶνος) was a boy's spinning-top, or the crest of a helmet; a _cylinder_ (κύλινδρος) was a roller; a _cube_ (κύβος) was a die: till these words were adopted by the geometers, and made to signify among them pure modifications of space. So an _angle_ (γωνία) was only a corner; a _point_ (σημεῖον) was a signal; a _line_ (γραμμὴ) was a mark; a _straight_ line (εὐθεῖα) was marked by an adjective which at first meant only _direct_. A _plane_ (ἐπίπεδον) is the neuter form of an adjective, which by its derivation means _on the ground_, and hence _flat_. In all these cases, the word adopted as a term of science has its sense rigorously fixed; and where the common use of the term is in any degree vague, its meaning may be modified at the same time that it is thus limited. Thus a _rhombus_ (ῥόμβος) by its derivation, might mean any figure which is _twisted_ out of a regular form; but it is confined by geometers to that figure which has four equal sides, its angles being oblique. In like manner, a _trapezium_ (τραπέζιον) originally signifies a _table_, and thus might denote any form; but as the tables of the Greeks had one side shorter than the opposite one, such a figure was at first called a _trapezium_. Afterwards the term was made to signify any figure with four unequal sides; a name being more needful in geometry for this kind of figure than for the original form.