4 



54(3 



GEOLOGY OF TUE BLACK HILLS. 



reference to Ilarney Peak and its longitude as given by the chronometers. 



This trianguhition was wholly performed by the topographer, Dr. McGilly- 



cuddy, in addition to his duties as topographer, and the true azimuth of a 



great many points in the Big Horn range ascertained ; but unfortunately 



our map will not be on a scale large enough to take in anytiiing beyond the 



Black Hills proper. 



3. Latitudes. 



Observations for latitude were made at every camp when the weather 

 was favorable, principally by double altitudes of Polaris ; while at the most 

 important camps, such as Harney Peak, Bear Lodge, and the junction of 

 the forks of the Cheyenne, by both north and south stars. During our 

 various trips up and down the numerous streams, circum-meridian observa- 

 tions of the sun were made daily, being rarely lost by cloudy weather. 

 So accurate were the courses run by the topographer that the differences 

 of latitude found by observation, and that platted from courses and dis- 

 tances, seldom varied more than a few hundred yards in a run of twenty- 

 five miles. 



My observations for time and latitude were all made with a sextant 

 attached to a small tripod of my own invention, weighing less than two 

 pounds. So convenient is this arrangement that the contact of the direct 

 and reflected image of a heavenly body can be observed with the same 

 accuracy as the passage over the wire of' a transit instrument. 



An observer who is familiar with the use of a sextant can by this 

 method easily obtain the latitude of his station to within one second of arc 

 in a single night, provided his instrumental errors have been carefully 

 ascertained, which unfortunately is seldom the case. Here the hands are 

 relieved from the weight of the sextant, which soon becomes tiresome, 

 especially with a heavy instrument. This contrivance permits the sextant 

 to move in azimuth without any sensible friction, while a delicate tangent 

 screw is used to bring the face of the instrument instantly into a vertical 

 plane. The sextant and clamp are instantly detached from the tripod and 

 readily replaced without aid of artificial light. 



The computations have been performed by the usual methods and need 

 not be detailed. The determined positions will be found in the table at 

 the end of the report. 



