554 



GEOLOGY OF THE BLACK HILLS. 



SKCTioN rv. 



SUPPLEMENT, 



In the summer of 1877 I was an assistant of R. J. Reeves, esq., United 

 States Astronomer and Surveyor, who had a contract for running so much 

 of the boundary Hne between Wyoming and Dakota as lies between the 

 j)arallels of 43°-45° of north latitude. The longitude of our starting-point 

 had been determined in 1874, and depended upon the longitude of Sid- 

 ney, Nebr., which had been established by telegraph. I think that our 

 starting-point can, therefore, be considered as very nearly correct. Our line 

 ran through the western portion of the Black Hills, and by estimate one 

 and one-quarter miles east of Camp Jenney, the longitude of which, as 

 given by my chronometers, is 103° 59' 48". west of Greenwich. Assum- 

 ing the correct longitude of our line to be 104° 03' 05". 85, the correct 

 longitude of Camp Jenney is 104° 04' 35". This will make the longitude 

 of Camp Jenney, as given by my chronometers, 4' 47" too far east; and as 

 the longitudes of all other places given in the table depend upon this 

 determination they will be in error to this amount. The meridians on our 

 large map of the Black Hills are made to conform to our boundary line 

 of 1877. 



Early in 1877 the telegraph line was extended to Deadwood, in the 

 northeast portion of the Black Hills, and later in the season Capt. W. S. 

 Stanton, Corps of Engineers, determined, by telegraphic signals from 

 Detroit, the longitudes of a great many places not only in the Hills, but of 

 places south nearly to Cheyenne. 



I quite agree with Capt. W. A. Jones, Corps of Engineers,* that the 

 use of box or marine chronometers for determining- longitudes in a moun- 

 tainous country is most unsatisfactory. It makes no difference how carefully 

 they may be handled their rates cannot be depended upon. 



One and perhaps the principal cause of the variations in the rates of 

 chronometers is the rapid changes in the height of the barometer in ascend- 



* Report of a Military Reconnoisance to Northwestern Wyoming in 1873. Washington, 1874. 



