Part II

. (I here give a _corrected text_.)

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. those of the 7 planets, the _firmamentum_ (_stellarum fixarum_), the _nonum coelum_, _decimum coelum_, _primum mobile_, and _coelum empyræum_.

18. See fig. 5, Pl. III. This is made upon the alt-azimuth system, and the plates are marked according to the latitude. The circles, called in Latin _circuli progressionum_, in Arabic _Almucantar[=a]t_, are circles of altitude, the largest imperfect one representing the horizon (_horizon obliquus_), and the central dot being the zenith, or pole of the horizon. In my figure, they are 'compounded by' 5 and 5, but Chaucer's shewed every second degree, i.e. it possessed 45 such circles. For the method of drawing them, see Stöffler, leaf 5, back.

19. Some Astrolabes shew 18 of these azimuthal circles, as in my figure (fig. 5, Pl. III). See Stöffler, leaf 13, where will be found also the rules for drawing them.

20. If accurately drawn, these _embelife_ or oblique lines should divide the portions of the three circles below the _horizon obliquus_ into twelve equal parts. Thus each arc is determined by having to pass through three known points. They are called _arcus horarum inequalium_, as they shew the 'houres inequales.'

21. In fig. 2, Pl. II, the _Rete_ is shewn as it appears when dropped into the depression in the front of the instrument. The shape of it varied much, and another drawing of one (copied from Camb. Univ. MS. Ii. 3. 3, fol. 66 _b_) is given in fig. 9, Pl. IV. The positions of the stars are marked by the extreme points of the metal tongues. Fig. 2 is taken from the figures in the Cambridge MSS., but the positions of the stars have been corrected by the list of latitudes and longitudes given by Stöffler, whom I have followed, not because he is _correct_, but because he probably represents their positions as they were supposed to be in Chaucer's time very nearly indeed. There was not room to inscribe the names of all the stars on the _Rete_, and to have written them _on the plate below_ would have conveyed a false impression. A list of the stars marked in fig. 2 is given in the note to § 21, l. 4. The Ecliptic is the circle which crosses the Equinoctial at its East and West points (fig. 2). In Chaucer's description of the zodiac, carefully note the distinction between the Zodiac of the Astrolabe and the Zodiac of Heaven. The former is only _six_ degrees broad, and shews only the northern half of the heavenly zodiac, the breadth of which is _imagined_ to be 12 degrees. Chaucer's zodiac only shewed _every other_ degree in the divisions round its border. This border is divided by help of a table of right ascensions of the various degrees of the ecliptic, which is by no means easily done. See Note on l. 4 of this section. I may add that the _Rete_ is also called _Aranea_ or _Volvellum_; in Arabic, _Al´ancab[=u]t_ (the spider).

22. _The Label._ See fig. 6, Pl. III. The _label_ is more usually used on the _front_ of the instrument, where the _Rete_ and other plates revolve. The _rule_ is used on the _back_, for taking altitudes by help of the scale.

23. _The Almury_; called also _denticulus_, _ostensor_, or 'calculer.' In fig. 2, it may be seen that the edge of the _Rete_ is cut away near the head of Capricorn, leaving only a small pointed projecting tongue, which is the almury or denticle, or (as we should now say) pointer. As the _Rete_ revolves, it points to the different degrees of the border. See also fig. 9, where the almury is plainly marked.

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