214 THE GREEK ASTRONOMY. 



munificence of Ptolemy Euergetes, two Armils, or 

 instruments composed of circles, which were placed 

 in the portico at Alexandria, and long used for 

 observations. If a circular rim were placed so 

 as to coincide with the plane of the equator, the 

 inner concave edge would be enlightened by the 

 sun's rays which came under the front edge, when 

 the sun was south of the equator, and by the rays 

 which came over the front edge, when the sun was 

 north of the equator : the moment of the transi- 

 tion would be the time of the equinox. Such an 

 instrument appears to be referred to by Hippar- 

 chus, as quoted by Ptolemy 8 . " The circle of cop- 

 per, which stands at Alexandria in what is called 

 the Square Porch, appears to mark, as the day of 

 the equinox, that on which the concave surface 

 begins to be enlightened from the other side." Such 

 an instrument was called an equinoctial armil. 



A solstitial armil is described by Ptolemy, con- 

 sisting of two circular rims, one sliding round 

 within the other, and the inner one furnished with 

 two pegs standing out from its surface at right 

 angles, and diametrically opposite to each other. 

 These circles being fixed in the plane of the meri- 

 dian, and the inner one turned, till, at noon, the 

 shadow of the peg in front falls upon the peg 

 behind, the position of the sun at noon would be 

 determined by the degrees on the outer circle. 



In calculation, the degree was conceived to 

 8 Ptol. Synt. iii. 2. 



