28 



A MANUAL OF TOPOGRAPHIC METHODS. 



difference of micrometer readings and levels are applied, with a small cor- 

 rection for differential refraction, and the result is the desired latitude. 



Following is an example of the reduction of six pairs of stars observed 

 for latitude at Rapid, South Dakota : 



LATITUDE DETERMINATION. 



Example of Reduction. 



[Station: Rapid, South Dakota. November 9, 1890. Half Rev. Micronieter=37.90u. One Div. Level. -.1.33.] 



November 9. Weighted mean = 44° 04' 45.59". 



OBSERVATIONS FOR TIME 



With the transit mounted, leveled, and adjusted in the meridian, the 

 chronograph set up and running and connected in a circuit with the battery, 

 and the chronometer and observing key connected in the same circuit the 

 observer is prepared to begin time observations. 



The lis) of stars which should be used is that given in the Berliner 

 Jahrbueh as the list is fuller and more accurate than that in any other cat- 

 alogue which gives day places. Stars should be so selected north and south 

 of the zenith that the azimuth errors will balance one another as nearly as 

 possible, as is explained hereafter. On the approach of the selected star to 

 the meridian, the telescope is set by means of the vertical circle upon the 

 altitude of the star above the horizon, deduced from the declination and the 

 latitude. As the star crosses each thread in the reticule, the fact is recorded 

 by pressing the observing key, which produces, as described above, a record 

 upon the chronograph sheet. In this way four time stars, as stars between 

 the equator and zenith are designated, and one circumpojar star, or a star so 

 near the pole that it is constantly in sight, should be observed. Then the 

 telescope should be reversed in the wyes and a similar set of stars observed. 



