ASTRONOMY. 35 



from the azimuth, the polar distance and the 

 complement of latitude, compute the altitude ; 

 the difference between this and the observed 

 altitude is the refraction. 



a. This may be easily done with the Astronomical 

 Circle, which measures altitudes and azimuths at 

 the same time. The circumpolar stars are well 

 suited to this observation. 



The triangle may be resolved by NAPIER'S rules, 

 Mauduit, p. 76. anot. 2. See other methods, 

 VINCE, 176., &c. WOODHOUSE, p. 85., &c. 

 LA LANDE, 2170., &c. 



I). If the time from the altitude of a known star, on 

 the one side of the meridian, to the same altitude 

 on the other side, be observed, the apparent alti- 

 tude may be calculated from thence ; and if a cir- 

 cumpolar star is used, great accuracy may be ob- 

 tained in this way, as a small change in the alti- 

 tude produces a great one in the horary angle ; 

 so that to observe this latter angle, is the most ex- 

 act way of coming to the knowledge of the appa- 

 rent altitude. 



48. The refraction varies with the state of the 

 barometer and thermometer, and, for the same 

 altitude, is nearly proportional to the density of 

 the air at the earth's surface. 



a. Dr 



