MATHEMATICAL AND PHYSICAL SCIENCE. 



[Diss. VI. 



Electrical 

 images. 



(879.) 

 Applica- 

 tion of the 

 theory of 

 magnet- 

 ism to 

 correct 

 ships' 

 compasses. 

 MM. Bar- 

 low and 

 Airy. 



son of Glasgow, for reducing the electrical effects of 

 electrified spheres upon points or other spheres with- 

 out them to those of electrified points in certain po- 

 sitions replacing the spheres, which points, from 

 certain analogies to well known optical formulae, 

 Professor Thomson has designated electrical images. 1 

 The mathematical theory of magnetism has re- 

 ceived a highly practical application in the correction 

 of the deviation of compass due to the local attrac- 

 tion of ships. Professor Barlow of Woolwich led the 

 way in attempting practically to correct the errors 

 thence arising. But when ships began to be con- 

 structed almost entirely of iron, the use of his " cor- 

 recting plate" was found to be totally insufficient. 

 To the Her. Dr Scoresby, practical navigation is 

 indebted for many ingenious observations on the 

 magnetism of ships, and suggestions as to the means 



of allowing for it ; while Mr Airy and Mr Archibald 

 Smith have the merit of applying the Theory of 

 Magnetism to the case in question. Mr Airy's in- 

 vestigations may be found in the Philosophical 

 Transactions for 1839 and 1855 ; from which it 

 appears that the effects of local magnetism in a ves- 

 sel may be speedily and effectually corrected for a 

 given place by the use of two permanent magnets 

 placed in rectangular positions relatively to the com- 

 pass ; and by the use of a mass of soft iron in a third 

 direction. But grave doubts still remain as to the 

 possibility of rendering such corrections permanent, 

 and applicable in all magnetic latitudes. A great 

 step has, however, been gained by putting the power 

 of readily verifying the compass corrections in any 

 part of the world in the hands of every intelligent 

 captain. 



8. Professor HANSTEEN Baron A. VON HUMBOLDT GAUSS Major-General SABINE Captain 

 Sir J. C. Ross. Progress of our Knowledge of Terrestrial Magnetism in the present Century. 



I could hardly have intentionally selected a more 

 characteristic example of the scientific progress of 

 the nineteenth century than the recent history of 

 terrestrial magnetism, even had it not accidentally 

 formed the closing section of this Dissertation. The 

 combination of extended methodical research in ob- 

 taining physical data, with mathematical skill in 

 comparing them and in deducing from them the 

 most important results, has been attended with 

 merited success. I regret that the unforeseen extent 

 of this historical sketch compels me to touch with 

 great brevity on the leading points of this research. 



Professor CHRISTOPHER HANSTEEN, of Christiania, 

 in Norway, is the person who has given pro- 

 bably the greatest impulse in recent times to 

 the efforts to methodize the facts and laws of 

 the earth's magnetism. M. Hansteen was born 

 26th September 1784, and is Professor of Astro- 

 nomy in the University of Christiania, and Direc- 

 tor of the Observatory. His dissertation, entitled 

 Magnetismus der Erde, published in 1819, which 

 received a prize from the Royal Danish Academy, 

 recapitulated all the authentic facts obtained by 

 voyagers and others from the earliest times. It 

 will be recollected 2 that Halley had represented 

 the magnetic variation at different parts of the 

 globe by lines traced on Mercator's chart, and 

 passing through all places where the variation 

 (or declination) of the needle from the true north 

 was equal ; and being well aware of the progressive 

 (or secular) changes in the course of these lines, he 



proposed the hypothesis of two pairs of magnetic 

 poles interior to the globe, of which* one pair re- 

 volves slowly. 



This hypothesis, little thought of at the time, (882.) 

 and perhaps of little value except as a help towards Halley's 

 the formal representation of the facts, appears to hyp? 1 ^ 818 - 

 have been revived by Wilcke, a Swede, whose la- charts, 

 bours attracted the attention of Professor Hansteen 

 first in 1807. M. Hansteen found the results of his 

 own collections to coincide well with Halley's chart 

 for 1700, and also with the hypothesis of four poles, 

 two in each hemisphere, one stronger than the other. 

 It results from these charts that the Line of No Fa- Line of No 

 riation, which, in 1600, formed a remarkable arch- Variation, 

 like curve, stretching from the Gulf of Mexico to 

 near the North Cape of Norway, then descending 

 through Central Europe to the Gulf of Guinea, had, 

 during the seventeenth and eighteenth centuries, be- 

 come gradually flattened (having passed through 

 Paris in 1669, and through London twelve years 

 earlier), and at present this part of the line of No 

 Variation is confined to the American continent and 

 neighbouring seas. Another and more complicated 

 branch of the same line traverses the Pacific Ocean, 

 making a complex serpentine track through East- 

 ern Asia and Siberia. The line of No Variation 

 may be expected to pass through those points of the 

 earth's surface towards which the needle converges, 

 which are sometimes called the magnetic poles (of 

 which more presently), and of which M. Hansteen 

 concludes the position to be as follows : 3 



1 Cambridge Mathematical Journal, 1850. 



2 See Fifth Dissertation, p. 741. 



3 From M. Hansteen's recent paper on the Secular Change of the Dip (Copenhagen, 1855). In this ingenious memoir it is in- 

 ferred, with considerable plausibility, that the annual diminution of the dip is decreasing, and consequently that a minimum 01 

 dip will occur in Europe before the close of this century. 



