448 



EXPEDITION TO POINT BARROW, ALASKA. 



of the Admiralty Manual of Scientific Inquiry, the Arctic Manual and Instructions, 1875, and 

 Aurora;, their character and spectra, by J. R. Capron, 1880. Also, with Terrestrial and Cosmical 

 Magnetism, by E. Walker, 18GC, and any other work they may require for their information." 



Besides the above paper, which is printed (pp. 12 to 14) in " Instructions No. 72, War Depart- 

 ment, Office of the Chief Signal Officer, Washington, D. C., June 17, 1881," the parties received 

 additional instructions Leaded (2) Obligatory observations in the domain of terrestrial magnetism, 

 and (3) Elective observations contained in the same order. Among these optional observations 

 are mentioned observations of tides and of earth currents; for both of these phenomena returns 

 were made. 



The Point Barrow party was also provided with a plan of the magnetic house, and received 

 the following note respecting the adjustment of the bifilar magetometer, which had been hastily 

 constructed from some remains of an older instrument: 



" The portable bifilar magnetometer. This instrument was reconstructed from such parts as 

 could be found from an old instrument. A collimator magnet was provided, also a new bifilar sus- 

 pension adjustable by means of a right and left handed screw in the place of a disk, as originally 

 supplied; the projecting arms indicating that the instrument had been arranged for an induction 

 inclinometer were removed. 



"It is to be used differentially or for variations only of the horizontal component of the mag- 

 netic force. The instrument is to be adjusted with the axis of the collimator magnet in the mag- 

 netic prime-vertical, and the variations of the horizontal force observed by readings of the scale. 



"If H= horizontal magnetic force, AH= variation of the same, v= angle of twist in the bifilar 

 suspension (usually between 40 and 70), Jr= variation of this angle (expressed in parts of 

 radius) then 



JH 



=cot 



Plan of magnetic 

 I f" 



p faint Barretr 



f 



^v. 



[ 



jk 

 i v 

 f 







\ 



\. 



"If K O = reading of the scale of any fixed part, say of the magnetic axis of the collimator, = 

 any reading at another time, a= value of one division of the scale in parts of radius (or angular 

 value in minutes times .000291), then J=(w w) a. 



