13. See fig. 3, Plate III. Each plate turns on a hinge, just like the 'sights' of a gun. One is drawn flat down, the other partly elevated. Each plate (_tabella vel pinnula_) has two holes, the smaller one being the lower. This _Rewle_ is named in Arabic _Alhidada_ or _Al´id[=a]da;_ in Latin _Verticulum_, from its turning easily on the centre; in Greek _Dioptra_, as carrying the sights. The straight edge, passing through the centre, is called the _Linea Fiduciæ_. It is pierced by a hole in the centre, of the same size as that in the _Mother_. 14. See fig. 4, Plate III. The _Pin_ is also called _Axis_ or _Clavus_, in Latin-Arabic _Alchitot;_ it occupies the position of the Arctic or North Pole, passing through the centre of the plates that are required to turn round it. The _Wedge_ is called _cuneus_, or _equus restringens_, in Arabic _Alfaras_ or the horse, because it was sometimes cut into the shape of a horse, as shewn in fig. 7, Plate IV, which is copied from MS. Univ. Camb. Ii. 3. 3. 15. See fig. 2, Plate II. In the figure, the cross-lines are partly hidden by the _Rete_, which is separate and removable, and revolves within the border. 16. The _Border_ was also called _Margilabrum_, _Margolabrum_, or _Limbus_. It is marked (as explained) with hour-letters and degrees. Each degree contains 4 minutes _of time_, and each of these minutes contains 60 seconds _of time_. 17. We may place under the _Rete_ any plates we please. If only the _Mother_ be under it, without any plate, we may suppose the _Mother_ marked as in fig. 2. The plate or disc (_tympanum_) which was usually dropped in under the _Rete_ is that shewn in fig. 5, Plate III, and which Chaucer now describes. Any number of these, marked differently for different latitudes, could be provided for the Astrolabe. The greatest declination of the sun measures the obliquity of the ecliptic, the true value of which is slightly variable, but was about 23° 31' in Chaucer's time, and about 23° 40' in the time of Ptolemy, who certainly assigns to it too large a value. The value of it must be known before the three circles can be drawn. The method of finding their relative magnitudes is very simple. Let ABCD (fig. 8, Pl. IV) be the tropic of Capricorn, BO the South line, OC the West line. Make the angle EOB equal to the obliquity (say 23½°), and join EA, meeting BO in F. Then OF is the radius of the Equatorial circle, and if GH be drawn parallel to EF, OH is the radius of the Tropic of Cancer. In the phrase _angulus primi motus_, _angulus_ must be taken to mean angular motion. The 'first moving' (_primus motus_) has its name of 'moving' (_motus_) from its denoting motion due to the _primum mobile_ or 'first moveable.' This _primum mobile_ (usually considered as the _ninth_ sphere) causes the rotation of the _eighth_ sphere, or _sphæra stellarum fixarum_. See the fig. in MS. Camb. Univ. Ii. 3. 3 (copied in fig. 10, Pl. V). Some authors make 12 heavens, viz.