ASTKONOMY AND ILVliOMETltlC UYrSOMETRY. 547 



SECTION II. 

 MAGNETIC VARIATION. 



The variation was frequently obtained by means of tlie needle of the 

 theodolite. Owing to the smallness of the circle surrounding the needle, 

 there was an error of reading of dr 5' in every reading, but the mean of 

 several readings was always taken. Great local disturbances were found at 

 one place on Box Elder Creek, and also on the summit of Inyan Kara. At 

 one spot on Box Elder the prismatic compass was entirely useless for some 

 miles, while at Inyan Kara so great a magnetic power did the rock possess 

 that a small fragment, when brought near the needle, would carry it com- 

 pletely round the circle. In preparing the table at the end of the report 

 all observations made in the vicinity of rock recognized as magnetic were 

 omitted. Ten stations in the Black Hills show a range from 15° 16' east to 

 15° 50' east, and give a mean of 15° 33.3' east. 



SECTION III. 

 BAROMETRIC WORK. 



The meteorological instruments were furnished by Mr. James Green, 

 and were of the best quality. They consisted of two mountain barome- 

 ters (mercurial), five aneroid barometers, and a number of thermometers. 



The mercurial barometers were read regularly in -our camps, but the 

 smallness of our party rendered it ordinarily impracticable for us to carry 

 them when we ascended peaks 'They were observed, however, on the 

 highest points of all, Harney and Terry Peaks. 



The deduced altitudes will be found in the table. The base station 

 was in each case the signal office at Cheyenne, and the computations were 

 according to the method and with the tables given by Guyot. The observa- 

 tions and computations are omitted except In the case of the peaks. 



HEIGHT OF HARNEY PEEK. 



Our barometrical base at Cheyenne was about one hundred and eighty 

 miles distant, but a glance at the weather-map on the day on whicli observa- 



