EXPEDITION TO POINT BARROW, ALASKA. 465 



PART III. DIFFERENTIAL MEASURES. 



HOURLY VARIATIONS OF THE DECLINATION, HORIZONTAL AND VERTICAL INTENSITIES, WITH 

 BI-MONTHLY TERM-DAY READINGS, AT UGLAAMIE, DECEMBER, 1881, TO AUGUST, 1883. 



I. The observations of the first year of occupation consist of hourly readings of the Fauth & 

 Co. magnetometer, Coast and Geodetic Survey No. 11 ; of the bifilar magnetometer, Coast and 

 Geodetic Survey No. 2; and of Dip Circle, Coast and Geodetic Survey No. 23, comprising vu>'in- 

 tions in the magnetic declination in the horizontal and in the total intensities between December, 

 1881, and September, 1882, together with term-day readings at the beginning and middle of each 

 mouth, as agreed upon for the Polar stations. There were four observers, viz : Sergt. James 

 Cassidy, Sergt. John Murdoch, Sergt. Middleton Smith, and A. C. Dark. They took regular 

 turns, each observing four hours at a time. Fifteen readings were taken each hour, five for 

 each instrument, viz, G minutes and 3 minutes before and after and at the full hour, commencing 

 with the declinometer and immediately followed by readings of the bifilar and dip instruments. 

 The temperature was noted. The presence of an aurora is indicated by an asterisk. 



The instrumental outfit of the second year of occupation being far more complete than that of 

 the first year, only so much of the record and discussion of the first year's work will be given here 

 as seems desirable; further consideration will be given to this year's record after the presentation 

 of the second year's work. 



II. The observations of the second year of occupation consist of hourly readings of the Brooke 

 magnetometers, comprising variations in the magnetic declination, in the horizontal intensity, and 

 in the -vertical intensity, between -September, 1882, and August, 1883, together with term day 

 readings on the 1st and 15th of each month, as agreed upon for the Polar stations. The observa- 

 tions were made by six observers, viz: Sergeants Murdoch and Smith and Mr. Dark, as in the 

 previous year, and Sergt. J. E. Maxfield, with Privates C. Ancor and J. Guzman. They took 

 watches of four hours each in regular rotation. Six readings were taken every hour, viz : The 

 horizontal force magnetometer was read 1 minutes before and again 1J minutes after the full 

 hour, the declinometer was read 1 minute before and 1 minute after, and the vertical force mag- 

 netometer minute before and \ minute after the full hour. The temperature was noted by two 

 thermometers suspended inside the cases or zinc covers of the horizontal force magnetometer and 

 of the declinometer. Suitable centigrade thermometers had been ordered, but they were not 

 received in time, and none was placed inside the case of the vertical force magnetometer. The 

 temperature of this magnet can be inferred from the mean of the readings of the thermometers of 

 the other instruments, which rarely deviated more than half a degree. The presence of an aurora 

 is indicated by an asterisk. 



ADJUSTMENT OF THE BROOKE DIFFERENTIAL MAGNETOMETERS. 



The unifilar magnetometer. The length of 1 division of the scale is 1 millimeter; the radins, 

 mirror to scale, is 1.719 meter; hence the angular value of 1 division of the scale=l'. 



(1) Observations for torsion coefficient, September 9, 1882, l b p. m. \Vhtn in the magnetic 

 H. Ex. 41 59 



