300 VHILOSOPHICAL TRANSACTIONS. [aNNO 1793. 



extremely scarce, and I know of only one copy in this kingdom, I take this op- 

 portunity of apprizing the curious, that it is to be met with in the British Mu- 

 seum, A short description however of the torquetum with a plate of the instru- 

 ment, will be found in Mons. Bailly's Astronomie Moderne, tome 1, p. 687; 

 and a description of the astrolabium armillare of Ptolemy, according to Regio- 

 montanus's conception of it, who may be considered as the best commentator on 

 the Almagest now to be met with, will be found in Weidler's Historia Astrono- 

 miae, quarto, 1741. 



The next author that presents himself is Copernicus, who lived in 1530, and 

 in his work De Revolutione Orbium coelestium, lib. 2, c. 14, De exquirendis 

 Stellarum Locis, professedly describes the same instrument with Ptolemy ; but, 

 as it appears to me, something more complicated, having a greater number of 

 circles, and in truth what in later times has been understood by the name, ar- 

 millary sphere. 



After Copernicus, I find, in a work of Apian, who was his contemporary, or 

 a little after him, viz. about 1338, a complete description of the torquetum, with 

 all the parts of it minutely detailed, assisted by 4 or 5 wooden plates, with the 

 use of the instrument. This work, which is also very scarce, is in folio, entitled, 

 Introductio geographica Petri Apiani in doctissimas Verneri Annotationes, &c. &c. 

 cui recens jam opera P. Apiani accessit Torquetum, Instrumentum pulcherrimum 

 sane et utilissimum. Ingolstadii, anno 1533. Towards the conclusion of this 

 work is a curious letter of Regiomontanus to Cardinal Bessarion, De Composi- 

 tione Meteoroscopii, that is, the armillary sphere that was used by Ptolemy, 

 with a plate of it. 



To Apian succeeded, at some distance but exceeded all that went before him, 

 the justly celebrated Tycho Brahe, who in his Astronomiae Instauratae Me- 

 chanica,* Noribergae, l6o2, folio, has given a description and wooden plates, of 

 no less than 4 different astrolabes, under the names of armillae zodiacales et 

 equatorise, of different sizes, from 4i to 10 feet diameter, divided into degrees 

 and minutes, and some of them into every 15 or 10 seconds, but furnished only 

 with plane sights. These large instruments were placed in towers appropriated 

 to each, with moveable roofs, one half of which was taken away at the time of 

 observation. A circumstance which it is curious to remark is, that Tycho, who 

 was attentive to every thing that could improve the accuracy of his observations, 

 made the axis of his 10 feet circle hollow, " Axis ejus e chalybe constans, et 

 undiqu^que apprime teres ; interius tamen cavus, ne pondere officiat, in diametro 

 est trium digitorum ;" a principle that has been very prudently re-adopted in these 

 later times, as will be presently seen. 



* See also Hist. Coelestis, lib. Prolegom. Tychonis Brahei, Augustse Vindelicorum, 1666, 2 vol. 

 folio, p. 118 and llj). — Orig. 



