DIVISIONS OP THE ZODIACK. 3 l^ 



sntv, be observed, the colure is turned, until the object 

 be seen in its proper place, on that secondary referred 

 to the ecliptick ; the circle representing the ecliptick, 

 being at the same time in the plane of the true ecliptick 

 and in its proper situation. Afterwards, the inner se- 

 condary is turned towards the moon (or to any star 

 intended to be observed), and the smaller circle within 

 it, bearing the two sights, is turned, until the moon 

 {or to any star intended to be observed), and the smaller 

 circle within it, bearing the two sights, is turned, until 

 the moon be seen in the line of the apertures. The 

 intersection of the secondary circle and ecliptick is the 

 place of the moon in longitude ; and the arc of the 

 secondary, between the aperture and the ecliptick, is 

 the latitude of the moon on either side (North or 

 South)." 



The same instrument, as described by Moxtucla 

 from the text of Ptolemy (1. 3. c. 2. *', consists of 

 six circles: first, a large circle representing the meri- 

 dian ; next, four circles united together, representing 

 the equator, ecliptick and two colures, and turning 

 within the first circle on the poles of the equator ; lastly, 

 a circle turning on the poles of the echptick, furnished 

 with sights and nearly touching, on its concave side, 

 the circumference of the ecliptick. 



The armillary sphere, described by the Arabian 



epitomiser, differs, therefore, from PTOLh^Mr's, in omit- 



,ting the equator and equinoctial colure, and adding an 



inner secondary of the ccHptick, which, as well as the 



meridian, is doubled. 



According to Lalande, the astrolabe of Ptg- 

 tEMY, from which Tycho Brake derived his equa- 

 torial armillary, consisted only of four circles-: two 

 placed at right angles to represent the ecliptick and 



* jyist. des Matbem. i. p. aOl. 



