The earliest sciences offer the earliest examples of technical terms. These are Geometry, Arithmetic, and Astronomy; to which we have soon after to add Harmonics, Mechanics, and Optics. In these sciences, we may notice the above-mentioned three different modes in which technical terms were formed.
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.
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], 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]; 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.
[1] Hist. Ind. Sci. b. iii. c. i.
[2] 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.
In later times, the limitation imposed upon a word by its appropriation to scientific purposes, is often more marked than in the cases above described. Thus the variation is made to mean, in astronomy, the second inequality of the moon’s motion; in magnetism, the variation signifies the angular deviation of the 261 compass-needle from the north; in pure mathematics, the variation of a quantity is the formula which expresses the result of any small change of the most general kind. In like manner, parallax (παράλλαξις) denotes a change in general, but is used by astronomers to signify the change produced by the spectator’s being removed from the center of the earth, his theoretical place, to the surface. Alkali at first denoted the ashes of a particular plant, but afterwards, all bodies having a certain class of chemical properties; and, in like manner, acid, the class opposed to alkali, was modified in signification by chemists, so as to refer no longer to the taste.
Words thus borrowed from common language, and converted by scientific writers into technical terms, have some advantages and some disadvantages. They possess this great convenience, that they are understood after a very short explanation, and retained in the memory without effort. On the other hand, they lead to some inconvenience; for since they have a meaning in common language, a careless reader is prone to disregard the technical limitation of this meaning, and to attempt to collect their import in scientific books, in the same vague and conjectural manner in which he collects the purpose of words in common cases. Hence the language of science, when thus resembling common language, is liable to be employed with an absence of that scientific precision which alone gives it value. Popular writers and talkers, when they speak of force, momentum, action and reaction, and the like, often afford examples of the inaccuracy thus arising from the scientific appropriation of common terms.
II. Another class of technical terms, which we find occurring as soon as speculative science assumes a distinct shape, consists of those which are intentionally constructed by speculators, and which contain some description or indication distinctive of the conception to which they are applied. Such are a parallelogram (παραλληλόγραμμον), which denotes a plane figure bounded by two pairs of parallel lines; a parallelopiped 262 (παραλληλοπίπεδον), which signifies a solid figure bounded by three pairs of parallel planes. A triangle (τρίγωνος, trigon) and a quadrangle (τετράγωνος, tetragon) were perhaps words invented independently of the mathematicians: but such words extended to other cases, pentagon, decagon, heccædecagon, polygon, are inventions of scientific men. Such also are tetrahedron, hexahedron, dodecahedron, tesseracontaoctohedron, polyhedron, and the like. These words being constructed by speculative writers, explain themselves, or at least require only some conventional limitation, easily adopted. Thus parallelogram, might mean a figure bounded by any number of sets of parallel lines, but it is conventionally restricted to a figure of four sides. So a great circle in a sphere means one which passes through the center of the sphere; and a small circle is any other. So in trigonometry, we have the hypotenuse (ὑποτενοῦσα), or subtending line, to designate the line subtending an angle, and especially a right angle. In this branch of mathematics we have many invented technical terms; as complement, supplement, cosine, cotangent, a spherical angle, the pole of a circle, or of a sphere. The word sine itself appears to belong to the class of terms already described as scientific appropriations of common terms, although its origin is somewhat obscure.
Mathematicians were naturally led to construct these and many other terms by the progress of their speculations. In like manner, when astronomy took the form of a speculative science, words were invented to denote distinctly the conceptions thus introduced. Thus the sun’s annual path among the stars, in which not only solar, but also all lunar eclipses occur, was termed the ecliptic. The circle which the sun describes in his diurnal motion, when the days and nights are equal, the Greeks called the equidiurnal (ἰσημερινὸς,) the Latin astronomers the equinoctial, and the corresponding circle on the earth was the equator. The ecliptic intersected the equinoctial in the equinoctial points. The solstices (in Greek, τροπαὶ) were the times when the sun arrested his motion northwards or 263 southwards; and the solstitial points (τὰ τροπικὰ σημεῖα) were the places, in the ecliptic where he then was. The name of meridians was given to circles passing through the poles of the equator; the solstitial colure (κόλουρος, curtailed), was one of these circles, which passes through the solstitial points, and is intercepted by the horizon.