Ixxxii ASTRONOMY. 



obviously be given by the mean of the azimuth circle readings for the two 

 observed directions. This process will thus give the direction of the meridian 

 free from the effect of any instrumental errors common to the equal altitudes 

 observed. Neither does it require any knowledge of the star's position (right 

 ascension and declination). It is therefore available to one provided with no- 

 thing but an instrument for measuring altitudes and azimuths, and is susceptible 

 of considerable precision when a series of such equal altitudes is carefully referred 

 to a terrestrial mark. 



When the sun is observed, it is essential to take account of its change in 

 declination between the first and the second observation. Let ^i and A^ be the 

 true azimuths counted from the meridian toward the east and west respectively 

 at the times /j and /j oi the two observations. Also, let AS be the increase in 

 declination of the sun in the interval (^2 — A)- Then 



A, -A,= ^« 



cos (f> sin h(f2 — A) 



Calling the azimuth circle readings for the east and west observations I^i and J?^ 

 respectively, the resulting azimuths are 



A, = Ki^2 - ^1) - UA - A), 



A2 = i(^2 - ^,) + h{A, - A,). 



Ref€7-ences. 



Many excellent treatises on spherical and practical astronomy are available. 

 Among these the most complete are the following : — 



" A Manual of Spherical and Practical Astronomy," by William Chauvenet. 

 Philadelphia : J. B. Lippincott & Co., 2 vols., 8vo, 5th ed., 1887. " A Treatise 

 on Practical Astronomy, as applied to Astronomy and Geodesy," by C. L. Doo- 

 little. New York: John Wiley & Sons, 8vo, 2d ed., 1888. " Lehrbuch der 

 Spharischen Astronomie," von F. Briinnow. Berlin : Fred. Diimler, 8vo, 185 1. 

 " Spherical Astronomy," by F. Briinno\v. Translated by the author from the 

 second German edition. London : Asher & Co., 8vo, 1865. 



