MAGNETIC OBSERVATIONS 



17 



That these changes in the circle reading cannot be ascribed to any motion of translation 

 of the pier M or the azimuth mark is evident from the small range in the azimuth determina- 

 tions and the fact that the readings after having reached a maximum returned to near the same 

 minimum. In the azimuth observations the Repsold circle was always pointed on, and bisected 

 the illuminated object glass of the magnetometer after this had been directed towards the 

 astronomic instrument. 



These readings, however, appear to be functions of the temperature and the close agree- 

 ment between the changes of temperature and changes of twist is graphically shown by curves 

 in the plate opposite on which the dates are represented by abscissae and the temperatures and 

 pointings on the mark are shown as ordinates. 



Program and Mbthods ok Observing 



A program for declination readings was arranged by Dr. ly. A. Bauer. With a view to 

 obtain a closed series every week, the observations were made to extend over four hours, these 

 periods being selected daily in succession according to the following detailed program, in civil 

 time, counting through 24 hours from midnight : 



In the beginning observers were changed, possibly more frequently than might have been 

 desired. Later, when they had become accustomed to the routine, the cold, and privations 

 which longer hours demanded, they willingly agreed to observe throughout 8 hours without 

 change. When observers changed during observations they observed alternately for 10 min- 

 utes or more. The observer is indicated by his initials, as follows : 



W. J. P., . . . W. J. Peters 



R. R. T R. R. Tafel 



J. v., .... John Vedoe 



R. W. P., . . . Russell Porter 



F.I/ Francis Long 



H. H. N., , . . Dr. Newcomb 



N. M. M. was an abbreviation used to denote that the incoming observer had been asked 

 previously to his first observation if he had divested himself of all magnetic materials. It 

 appears at every change of observers in the original record, but is left out of the published 

 notes for lack of space. 



Observations were made at every two minutes. Sometimes one would be accidentally 

 taken later than planned, in which case the tenths of a minute were noted. 



