Today | News | Books | Recipes Adventure | Science Fiction | Ghost stories | Poetry | Children | History Bookor 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. 4. See fig. 1; Chaucer describes the 'bak-half' of the instrument first. The centre of the 'large hole amydde' is the centre of the instrument, where a smaller hole is pierced completely through. The _Southe lyne_ (marked _Meridies_ in figs. 1 and 2) is also called _Linea Meridiei;_ the _North lyne_ is also named _Linea Mediæ Noctis_. 5. The _Est lyne_ is marked with the word _Oriens;_ the _West lyne_, with _Occidens_. 6. The rule is the same as in heraldry, the _right_ or _dexter_ side being towards the spectator's left. 7. As the 360 degrees answer to 24 hours of time, 15° answer to an hour, and 5° to twenty minutes, or a _Mile-way_, as it is the average time for walking a mile. So also 1° answers to 4 minutes of time. See the two outermost circles in fig. 1, and the divisions of the 'border' in fig. 2. 8. See the third and fourth circles (reckoning inwards) in fig. 1. 9. See the fifth and sixth circles in fig. 1. 10. See the seventh, eighth, and ninth circles in fig. 1. The names of the months are all Roman. The month formerly called _Quinctilis_ was first called _Julius_ in B.C. 44; that called _Sextilis_ was named _Augustus_ in B.C. 27. It is a mistake to say that Julius and Augustus made the alterations spoken of in the text; what Julius Cæsar really did, was to add 2 days to the months of January, August (Sextilis), and December, and 1 day to April, June, September, and November. February never had more than 28 days till he introduced bissextile years. 11. See the two inmost circles in fig. 1. The names given are adopted from a comparison of the figures in the Cambridge University and Trinity MSS., neither of which are quite correct. The letters of the 'Abc.' are what we now call the Sunday letters. The festivals marked are those of St. Paul (Jan. 25), The Purification (Feb. 2), The Annunciation (Mar. 25), The Invention of the Holy Cross (May 3), St. John the Baptist (June 24), St. James (July 25), St. Lawrence (Aug. 10), The Nativity of the Blessed Virgin (Sept. 8), St. Luke (Oct. 18), St. Martin of Tours (Nov. 11), and St. Thomas (Dec. 21). 12. The 'scale' is in Latin _Quadrans_, or _Scala Altimetra_. It is certain that Chaucer has here made a slip, which cannot be fairly laid to the charge of the scribes, as the MSS. agree in transposing _versa_ and _recta_. The side-parts of the scale are called _Umbra versa_, the lower part _Umbra recta_ or _extensa_. This will appear more clearly at the end of 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 point |