106 THIRD REPORT — 1833. 



I. The Direction of the Terrestrial Magnetic Force. 



1. The Variation of the Needle. — For some centuries after 

 the directive property of the loadstone was discovered, it was 

 generally supposed that the needle pointed correctly towards 

 the pole of the heavens. It has however been said, on the 

 authority of a letter by Peter Adsiger, that the variation of 

 the needle was known as early as 1269; and if we fully admit 

 the authenticity of this letter, we must allow that the writer 

 was at that date not only aware of the fact, but that he had 

 observed the extent of the deviation of the needle from the 

 meridian*. It is possible that such an observation as this 

 may have been made at this early period by an individual de- 

 voting his time to the examination of magnetical phaenomena; 



* This curious and highly interesting letter, dated the 8th of August 1269, 

 is contained in a volume of manuscripts in the Library of the University of 

 Leyden, and we are indebted to Cavallo for having published extracts from it. 

 The variation is thus referred to : " Take notice that the magnet (stone), as well 

 as the needle that has been touched (rubbed) by it, does not point exactly to 

 the poles ; but that part of it which is reckoned to point to the south declines a 

 little to the west, and that part which looks towards the north inclines as much 

 to the east. The exact quantity of this declination I have found, after numer- 

 ous experiments, to be five degrees. However, this declination is no obstacle 

 to our guidance, because we make the needle itself decline from the true south 

 by nearly one point and an half towards the west. A point, then, contains five 

 degrees." (Letter of Peter Adsiger, Cavallo On Magnetism, London 1 800, p. 317.) 

 It is certainly extraordinary, if so clear an account of the deviation of the needle 

 from the meridian as this, was communicated to any one by the person who had 

 himself observed that deviation, that for more than two centuries afterwards we 

 should have no record of a second observation of the fact. This alone would 

 throw doubt on the authenticity of the letter, and the estimate given of the 

 variation may appear to confirm these doubts ; for, according to the period of 

 change which best agrees with the observations during more than two hundred 

 years, the variation, if observed, would have been found to be westerly instead 

 of easterly in 1269. It may however be urged, that as the whole period of 

 change has not yet elapsed since observations were made, we are not in pos- 

 session of a sufficient number of facts to authorize us to draw conclusions re- 

 specting the variation at such an early date ; and also, that if the letter be spu- 

 rious, or the original date have been altered to that which it bears, this or the 

 fabrication can only have been for the purpose of founding claims in consequence 

 of the contents of this letter ; and as no such claims have been advanced, there 

 appears no motive either for fabrication or alteration. In a preceding part of 

 the letter the author gives methods for finding the poles of a loadstone ; and 

 certainly the direction of the axis could not be determined to within five degrees 

 by either of these ; so that, as regards the loadstone, we may, I think, conclude 

 that the author did not make the observation. As a matter of curious history 

 connected with magnetism, it is desirable that either the authenticity of this 

 letter should be clearly established, or reasons given for doubting it, by those 

 who have an opportunity of cousulting the original. 



