VERTICAL FOECE BASE-LINE VALUES. 



83 



In view of what has licon already said and of the difference between the Declination results under 

 (i) and (ii) in Table VIII, it is, perhaps, unwise to say more than that it is desirable that the attention of 

 the observers of the next Antarctic Expedition should be called to the importance of making a careful 

 study of the annual inequality. If an inequality with a range of the order suggested by Table VIII 

 should be established, it would be a most important result, strongly suggestive of an annual oscillation 

 in the position of the S. Magnetic pole. 



The question may be asked why a second set of results answering to those in Table VII and to those 

 under (ii) in Table VIII has not been given for the Horizontal Force. The reason is simply that the 

 Horizontal-Force base line inevitably changes with time as the moment of the suspended magnet alters, 

 and there is the further reason that a discontinuity arose more than once through breakage of the 

 suspension, or similar disturbing cause. A diminution in the moment at the rate of 1 per cent, per annum 

 would have had at Winter Quarters the same apparent effect as a secular change of 65y per annum, and 

 it is by no means improbable that the change of moment amounted to several per cent, as the magnet was 

 exposed to large and numerous changes of temperature. 



17. Table IX shows the Vertical-Force base-line values observed (i.e. derived by combining observed 

 values of dip with the corresponding values given by the Horizontal-Force curves) and those actually 

 employed. As already explained, the probable error in the value of V (Vertical Force) derived from a 

 single observation of dip is very large. This made it advisable to derive base-line values from the 

 observations of a number of months combined, when this appeared feasible, allowing for apparent curve 

 discontinuities. Thus the base-line values up to September 12, 1902, were determined by combining the 

 absolute observations of dip made in April, May, June, July and September. The base-line values from 

 October 1 to November 12 depend on the absolute observation of October alone. The base-line values for 

 the latter part of November, 1902, and up to the end of January, 1903, depend on the observation of 

 December. The base-line value for February, 1903, is from the absolute observation of that month. From 

 March to October, 1903, the base-line values depend on the observations of March, May, June, August and 

 September combined, allowing for apparent curve discontinuities. The base-line values for November, 

 1903, and January, 1904, depend on the absolute observations of these respective months, and the base-line 

 value for December, 1903, was interpolated. 



It is obvious from what has been already stated that the Vertical-Force base lines for individual months 

 are affected by uncertainties which would render any deductions as to secular change or annual inequality 

 of very problematical value. 



TABLE IX. Vertical-Force Base-Line Values. 



* Of the two values given for November, 1902, the firt was applied up to and including the 12th, the second for the rest 

 of the month. 



M 2 



