100 ON THE REPEATING REFLECTING CIRCLE. 



there is nothing else to be attended to than observing 

 the rules stated in articles 4 and 5. 



7. Calculate the variation in declination for the quantity 



corresponding to the sums No. 4. This may either be 

 done simply by proportions, worked by logarithms, or 

 taken from Tables calculated for the purpose. 



8. Form the apparent declination which will be equal to the 



true declination + (the refraction — parallax) + the 

 variation in declination for the difference of meri- 

 dians, + the variation of declination for the middle of 

 the series that you propose to calculate. If the series 

 is composed of 10, 20, 30, or 40 observations, the re- 

 sult will be the y^th, aV^h, sV^h, or ^^^th of the quan- 

 tity indicated by the sum No. 4. The sign + is appli- 

 ed when these quantities augment the declination, and 

 the sign — when they diminish it. 



9. Prepare the constant factor 



Cosine apparent declination. Cosine latitude 



Sine apparent zenith distance 



10. Add the logarithm of the constant factor to the loga- 

 rithm of the sum No. 5., which gives the logarithm of 

 the meridian correction. 



11. Subtract the correction from the apparent zenith dis- 

 tance. 



12. To the result of No. 11. add or subtract the apparent 



declination (No. 8.) which gives the distance from the 

 zenith to the equator, or the Latitude of the place of 

 observation. 



It is almost unnecessary to add, that my artificial horizon 

 has always been quicksilver, (when the situation would permit 

 me to us it), covered with one of Troughton's glass roofs, as I 



have 



