Furthermore, yif thou wilt knowe in _umbra versa_, by the craft of _umbra recta_, I suppose thou take the altitude at the poynt of 4, and makest a marke; and thou goost neer til thou hast it at the poynt of 3, and than makest thou ther a-nother mark. Than muste thou devyde 144 by eche of the poyntes be-fornseyd, as thus: yif thou 5 devyde 144 be 4, and the nombre that cometh ther-of schal be 36, and yif thou devyde 144 be 3, and the nombre that cometh ther-of schal be 48, thanne loke what is the difference be-tween 36 and 48, and ther shalt thou fynde 12; and right as 12 hath him to 12, right so the space be-tween two prikkes hath him to the altitude of the thing. 10 COMMENTARY ("FOOTNOTES"). Little Lewis my son, I perceive that thou wouldst learn the Conclusions of the Astrolabe; wherefore I have given thee an instrument constructed for the latitude of Oxford, and purpose to teach thee _some_ of these conclusions. I say _some_, for three reasons; (1) because some of them are unknown in this land; (2) because some are uncertain; or else (3) are too hard. This treatise, divided into five parts, I write for thee in English, just as Greeks, Arabians, Jews, and Romans were accustomed to write such things in their own tongue. I pray all to excuse my shortcomings; and thou, Lewis, shouldst thank me if I teach thee as much in English as most common treatises can do in Latin. I have done no more than compile from old writers on the subject, and I have translated it into English solely for thine instruction; and with this sword shall I slay envy. The _first_ part gives a description of the instrument itself. The _second_ teaches the practical working of it. The _third_ shall contain tables of latitudes and longitudes of fixed stars, declinations of the sun, and the longitudes of certain towns. The _fourth_ shall shew the motions of the heavenly bodies, and especially of the moon. The _fifth_ shall teach a great part of the general rules of astronomical theory. Here begins the _first_ part; i.e. the description of the Astrolabe itself. 1. _The Ring._ See figs. 1 and 2. The Latin name is _Armilla suspensoria_; the Arabic name is spelt _alhahuacia_ in MS. Camb. Univ. Ii. 3. 3, but Stöffler says it is _Alanthica_, _Alphantia_, or _Abalhantica_. For the meaning of 'rewle,' see § 13. 2. _The Turet._ This answers nearly to what we call an _eye_ or a _swivel_. The metal plate, or loop, to which it is fastened, or in which it turns, is called in Latin _Ansa_ or _Armilla Reflexa_, in Arabic _Alhabos_. 3. _The Moder._ In Latin, _Mater_ or _Rotula_. This forms the body of the instrument, the back of which is shewn in fig. 1, the front in fig. 2. The 'large hole' is the wide depression sunk in the front of it, into which the various discs are dropped. In the figure, the 'Rete' is shewn fitted into it.