28 THE WORLD. 



Mercury around the earth, the planet however, instead of revolving 

 in this circle, was supposed to be revolving in another smaller 

 circle c a b d, whose centre v was carried forward as the circle A 

 B C revolved around the earth, in the order of the letters, the 

 planet moving in the contrary direction in the small circle c a b d 

 would apparently describe the curve line d e f g h, being sta- 

 tionary at / and h, and apparently moving backward through the 

 arch f g h. Now in order to make Venus and Mercury always 

 accompany the sun, the centre v of the small circle, was supposed 

 to be always in a right line nearly, between the earth and sun. 

 Such was the Ptolemaic system, and as it appeared to explain the 

 irregular motions by really uniform, or true circular motions, it was 

 soon adopted as the true system of the world. In the time of 

 Ptolemy astronomical instruments began to be used; for some 

 time previous however, the eastern nations, in order to ascertain 

 the instant of mid-summer, or mid-winter, had been in the habit 

 of measuring the length of the shadow of a vertical gnomon or 

 style, but Ptolemy introduced the use of graduated spheres. We 

 have already observed that all circles are divided into 360 degrees, 

 and these subdivided into 60 minutes each. Hence it is evident 

 that by means of a graduated circle, angular distances may be 

 measured in the sky. An angle, it must be remembered, is simply 

 the inclination of two lines and has no reference at all to the 

 length of the lines, thus S A B is the angular distance of the star 



S from the object B. To observe this angle, or inclination, we 

 may use a small graduated circle thus. Lot A C D be a circle 

 graduated into 360, having a moveable index turning on its, 

 centre, which index is furnished at each end with a sight-hole. 

 First look with the index towards the object B, and observe the 



