3o6 



SCIENTIFIC RESULTS OF ZIEGLER POLAR EXPEDITION 



Annuai, Variation in Magnetic Declination 



By means of the preceding value of the annual rate of secular variation in magnetic declina- 

 tion at Teplitz Bay the monthly mean hourly declinations and means were reduced to epoch 

 1904.0 as per tabulation on page 288. By the use of Bessel's periodic function in the particulair 

 case of a cycle of twelve equidistant observations the following expression has been obtained^ 

 after the methods detailed on page 290, as representing the annual variation : 



D= 22° 38.'5 + 9.'27sin((9+244° 59') + 2/32 sin (2^+ 141° 07') 

 + 1/41 sin (3 <? + 4° 45') ± 0/33* 



In this formula the angle counts from the middle of June as 0°, no account being taken 

 of the inequality in length of months. The addition of a fourth term results in no improve- 

 ment, as in this case the probable error is identical with that above. It may be noted that the 

 range of variation on the mean of year is unusually large, the maximum deflection early in 

 December being + 10. '2 while the opposite extreme about the middle of August is — 10. 'o. 

 The computed and observed values are shown graphically to scale in figure 20. The following 

 summary of variations on the mean yearly value will serve further to indicate the agreement " 

 between the observed (O) and computed (C) quantities, easterly deviations being denoted by 

 a plus sign, and vice versa : 



Month 



June 



July 



August . . . 

 September 

 October. . . 

 November. 



Month 



December. 

 January. . , 

 February . 

 March ... 



April 



May 



*The probable error of a single representation is d=o.'33. 



