TERUESTRIAL MAGNETISM. 223 



ence of the accidental anomalies of the magnetic force itself, and 

 that they do not correspond to the same year* ; and, on the 

 other hand, that our formula? do not include members beyond 

 the fourth order, whereas those of the following order may still 

 be very sensible. When due weight is allowed to these circum- 

 stances, the agreement between calculation and experiment ap- 

 pears to be as satisfactory as we are entitled to expect from a first 

 attempt. 



V 



As our expression for ^ may therefore be safely regarded as 



coming near the truth, at least in its more important members, it 

 has appeared worth while to form a graphical representation of 

 the course of the numerical values of this function. This 

 has been done in a map drawn by Dr. Goldschmidt, in three 

 parts, the first on Mercator's projection, passing round the 

 globe, and including all the parallels between 70° north, and 

 70° south lat. ; the other two being polar projections, extend- 

 ing to lat. 65°. The corrections and additions which will arise 

 from a fresh calculation resting on more perfect data, may, doubt- 

 less, cause material alterations of position in these lines, parti- 

 cularly in the high southern latitudes ; but no important change 

 in the whole form of the system of lines can be supposed without 



V 



such alterations in the expression for ^ as would destroy the 



agreement with existing observations. We are thus led to the 

 important result, that the system of lines of equal values of F, 

 on the surface of the earth, is actually comprehended by the 

 simplest type described in Art. 13, and that consequently there 

 are on the earth only two magnetic poles, apart from the possible 

 case of local exception spoken of in Art. 13. 



* The last article presents instances of discordances between different ob- 

 servers at one and the same place ; I will notice some others, which are much 

 greater than can with any degree of probability be attributed to yearly changes. 

 The dip at Valparaiso was, in 1829, according to King, 40° 11'; in 1835, ac- 

 cording to Fitz Roy, 38° 3'. In Mauritius the intensity was 1-096 in 1 818, ac- 

 cording to Freycinet, and M92 in 1836, according to Fitz Roy. The difference 

 is still greater at Otaheite, where Erman's intensity = 1-172 in 1830, and 

 Fitz Roy's, in 1835, = 1-017. Otaheite is a station of the highest importance 

 for the future improvement of the elements : the difference between the two 

 determinations made there by different observers, considerably exceeds the 

 greatest difference between the computed and observed intensities in our eighty- 

 six comparisons. 



P 2 



