INTRODUCTION 13 



direction of the shadow in order to conveniently reckon 

 the time. 1 



Besides the gnomon, the Chaldeans used the polos. 

 The polos is a half sphere hollowed out in a block of 

 stone or metal, at the bottom of which is fixed a style 

 with its end reaching exactly to the centre of the 

 sphere. 2 Hence the name oxdcprj (boat) given by the 

 Greeks to the polos. By this means an exact repre- 

 sentation of the sun's movement was obtained ; " the 

 shadow of the point of the style moves in the interior 

 of the polos as the sun moves in the heavens, in the 

 same direction and with the same angular velocity at 

 each instant ; it is only the sense of the motion which is 

 reversed." 3 The hourly division of the time, repre- 

 sented by the meridians of the hemisphere, remains 

 the same for all periods of the year. The shadow, in its 

 curved path, sweeps a zone in latitude, whose breadth 

 corresponds to the difference between the shadows pro- 

 jected at the summer and winter solstices. 



To measure time during the night the Chaldeans at 

 first used the clepsydra, and it is probably by means of 

 this instrument that they divided the zodiac into 12 

 equal regions. 4 



Later, by combining the polos with a kind of 

 armillary sphere, they could verify their nocturnal 

 measurements in the following manner : 5 Imagine an 

 open-work sphere, made of strips of metal for instance, 

 representing the celestial sphere, and more especially 

 the zodiacal zone with its principal constellations, and 

 let this sphere be constructed in such a manner that it 

 can move within the polos and be exactly adjusted to 

 it. Suppose the zodiac to be divided into 360 degrees, 



1 2 Bigourdan, Astronomie, p. 92 et seq. 

 9 24 Sageret, Systeme, p. 106. 



3 Ibid. 



4 2 Bigourdan, Astronomie, p. 95. 



5 25 Tannery, Science helUne, p. 84, 



