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. Part II, § 1. [The Latin headings to the propositions are taken from the MS. in St. John's College, Cambridge.] See fig. 1. Any straight edge laid across from the centre will shew this at once. Chaucer, reckoning by the old style, differs from us by about eight days. The first degree of Aries, which in his time answered to the 12th of March, now vibrates between the 20th and 21st of that month. This difference of eight days must be carefully borne in mind in calculating Chaucer's dates. 2. Here 'thy left side' means the left side of thine own body, and therefore the right or Eastern edge of the Astrolabe. In taking the altitude of the sun, the rays are allowed to shine through the holes; but the stars are observed by looking through them. See figs. 1 and 3. 3. Drop the disc (fig. 5) within the border of the mother, and the _Rete_ over it. Take the sun's altitude by § 2, and let it be 25½°. As the altitude was taken by the _back_ of the Astrolabe, turn it over, and then let the _Rete_ revolve westward till the 1st point of Aries is just within the altitude-circle marked 25, allowing for the ½ degree by guess. This will bring the denticle near the letter C, and the first point of Aries near X, which means 9 A.M. At the same time, the 20th degree of Gemini will be on the _horizon obliquus_. See fig. 11, Pl. V. This result can be approximately verified by a common globe thus; elevate the pole nearly 52°; turn the small brass hour-circle so that the figure XII lies on the equinoctial colure; then turn the globe till IX lies under the brass meridian. In the next example, by the Astrolabe, let the height of Alhabor (Sirius) be about 18°. Turn the denticle Eastward till it touches the 58th degree near the letter O, and it will be found that Alhabor is about 18° high among the _almicanteras_, whilst the first point of Aries points to 32° near the letter H, i.e. to 8 minutes past 8 P.M.; whilst at the same time, the 23rd degree of Libra is almost on the _Horizon obliquus_ on the Eastern side. By the globe, at about 8 minutes past 8 P.M., the altitude of Sirius is very nearly 18°, and the 23rd of Libra is very near the Eastern horizon. See fig. 12, Pl. V.