Electrical 
images. 
(879.) 
Applica- 
tion of the 
theory of 
magnet- 
ism to 
correct 
ships’ 
compasses, 
MM. Bar- 
low and 
Airy. 
(880.) 
Recent 
history of 
terrestrial 
magnet- 
ism. 
(881.) 
Professor 
Hansteen 
— Magne- 
tismus der 
Erde. 
Variation 
or decli- 
nation, 
990 
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 formula, 
Professor Thomson has designated electrical images.' 
The mathematical theory of magnetism has re- 
eeived 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 Rev. Dr Scoresby, practical navigation is 
indebted for many ingenious observations on the 
magnetism of ships, and suggestions as to the means 
MATHEMATICAL AND PHYSICAL SCIENCE. 
(Diss. V1. 
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 HumpoLtpr—Gauss—Major-General Sasrne—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 CuristoPpHER 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 1783, 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? 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, 
and perhaps of little value except as a help towards Halley’s 
the formal representation of the facts, appears to 
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 Va- Line of No 
riation, whieh, in 1600, formed a remarkable arch- Vriation. 
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 Hast- 
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 :?>— 
1 Cambridge Math ical Journal, 1850. 
2 See Fifth Dissertation, p. 741. 
% 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 of 
dip will occur in Europe before the close of this century. 
