bearing to the nearest 5' or 10'. Each scale is designated at its 

 proper radial distance on the reading arm. To use the appliance: 

 First, the altitude is set against the latitude on scale a. 



Opposite the index, read number A. 



Then, the altitude is set against the latitude on scale b. 

 Opposite the given declination, read number B. 

 Finally, Opposite the number A + B read the true bear- 

 ing of the sun. 



The scale A is arranged to solve tan Alt. X tan Lat. 

 The scale B is arranged to solve sec Alt. X sec Lat. X sin Dec. 

 A -f- B = cos bearing. 



This arrangement evidently solves the elemental cosine law of 

 spherical trigonometry involved in the third formula on p. 154 



Fig. 91 



The basic advantage of this appliance lies in the magnification 

 and legibility of the scales. On the outer 7-inch circle, the spaces 

 are as open as one might expect to find the divisions, on a protractor 

 5 feet across. It is claimed that the accuracy of the Meridiograph 

 keeps pace with that of observation. A reading can be taken to 

 the nearest minute on the assumption that the user will be able to 

 interpolate a space of >oo inch. The sufficiency of this allowance 

 has been checked on numerous sets of data. 



156 



