ence of the accidental anomalies of the magnetic force itself, and that they do not correspond to the same year[1]; and, on the other hand, that our formulæ 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 circumstances, the agreement between calculation and experiment appears to be as satisfactory as we are entitled to expect from a first attempt.
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, extending to lat. 65°. The corrections and additions which will arise from a fresh calculation resting on more perfect data, may, doubtless, cause material alterations of position in these lines, particularly in the high southern latitudes; but no important change in the whole form of the system of lines can be supposed without 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 , 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 observers 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, according to Fitz Roy, 38° 3′. In Mauritius the intensity was 1·096 in 1818, according to Freycinet, and 1·192 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
