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. We have borrowed from the Arabians various astronomical terms, as _Zenith_, _Nadir_, _Azimuth_, _Almacantar_. And these words, which among the Arabians probably belonged to the first class, of appropriated scientific terms, are for us examples of the second class, invented scientific terms; although they differ from most that we have mentioned, in not containing an etymology corresponding to their meaning in any language with which European cultivators of science are generally familiar. Indeed, the distinction of our two classes, though convenient, is in a great measure, casual. Thus most of the words we formerly mentioned, as _parallax_, _horizon_, _eclipse_, though appropriated technical terms among the Greeks, are to us invented technical terms. In the construction of such terms as we are now considering, those languages have a great advantage which possess a power of forming words by composition. This was eminently the case with the Greek language; and hence most of the ancient terms of science in that language, when their origin is once explained, are clearly understood and easily retained. Of modern European languages, the German possesses the greatest facility of composition; and hence scientific authors in that language are able to invent terms which it is impossible to imitate in the other languages of Europe. Thus Weiss distinguishes his various systems of crystals as _zwei-und-zwei-gliedrig_, _ein-und-zwei-gliedrig_, _drey-und-drey-gliedrig,_ _&c._, (two-and-two-membered, one-and-two-membered, &c.) And Hessel, also a writer on crystallography, speaks of _doubly-one-membered edges_, _four-and-three spaced rays_, and the like.