i.e. They pass the seven planetary spheres; then the sphere of fixed stars; then the crystalline or transparent one, whose swaying motion or libration measures the amount of the precession and nutation so often talked of; and then, the sphere of the _primum mobile_ itself. But Milton clearly himself believed in the Copernican system; see Paradise Lost, viii. 121-140, where the _primum mobile_ is described in the lines-- 'that swift Nocturnal and diurnal rhomb supposed, Invisible else above all stars, the wheel Of day and night.' § 18. 8. _compowned by 2 & 2._ This means that in the _best_ astrolabes, _every_ almicantarath for every degree of latitude was marked; as may be seen in Metius. In others, including the one given by Chaucer to his son, they were marked only for every other degree. See Part II. sect. 5, l. 2. § 19. 7. _cenith_, as here used, has a totally different meaning from that of _senith_, in l. 1 above. The _senith_ in l. 1 is what we still call the _zenith_; but the _cenith_ in l. 7 means the point of the horizon denoting the sun's place in azimuth. Contrary to what one might expect, the _latter_ is the true original meaning, as the word _zenith_ is corrupted from the root of the word which we now spell _azimuth_. The Arabic _as-sant_ is a way or path; _al-samt_, a point of the horizon, and, secondly, an azimuthal circle. The plural of _al-samt_ is _assum[=u]t_, whence _azimuth_. But _zenith_ is a corruption of _semt_, from _samt al-r[=a]s_, the Arabic name of the vertex of heaven (_r[=a]s_ meaning _a head_); and the qualifying _al-r[=a]s_, the most important part of the phrase, has been improperly dropped. So far from the reading _cenith_ being wrong here, it is most entirely right, and may be found (better spelt _cenit_) in the same sense in Messahala. See p. 213, second footnote. For _cenith_, some late copies have _signet_, evidently taken from the Latin word _signum_. They make the same mistake even in l. 12 of section 18. § 21. 4. _sterres fixes_, fixed stars; here the _s_ again appears in a plural adjective of French derivation; see note above, to § 5. 8. In MSS. Ii. 3. 3 and Ii. 1. 13 in the Cambridge University Library, is an interesting list of the 49 stars most usually placed upon the Astrolabe. The stars which are represented by the points of the tongues in Fig. 2 are the same as those in the diagram from which Fig. 2 is copied, the original of which is in MS. A. I have slightly altered the positions of the points of the tongues, to make them somewhat more correct. The following is the list of the stars there shewn; most of their names are written in the MS. Cf. footnote on p. 186.