> 
D Sy. an. 2, 187 3] ; 
_true magnetic poles has not always been attended to with suffi- 
cient care, and this is partly to be accounted for by the con- 
siderable and often preponderating influence of these maximum 
points on the several magnetic elements. A consequent doubt 
was for a long time entertained respecting the number of the 
magnetic poles. Halley, from a careful study of an extensive 
series of declination observations, made partly by himself in 
1698-9, was led to the conclusion that the earth has four magnetic 
poles. The same opinion has’been most ably advocated within 
our own days by Prof. Hansteen of Christiana, who had pre- 
viously collected together a vast mass of observations of the 
declination, dip, and horizontal force. But the interesting series 
of results which Hansteen has brought forward in support of his 
view, are most readily explained by the evident changes that 
have taken place in the magnetic state of the region of maximum 
intensity situated in Siberia, where Hansteen himself specially 
observed. The Northern regions, where the magnetic force is 
greatest, abound in ferruginous strata, and there too the intensity 
of the cold far exceeds anything that is experienced in other lands 
on the same parallel of latitude. These regions may therefore 
not only be charged with a most abundant supply of permanent 
magnetism, but they may also be affected to a very considerable 
degree by atmospheric changes, and by those electric currents 
that are continually passing to and fro in the upper crust of the 
earth, and are doubtless producing very important changes in the 
subpermanent magnetism of certain layers of softer ferruginous 
matter. Whatever may be the nature of terrestrial magnetism, 
we cannot ignore the great influence exercised on its distribution 
by what may be termed local magnetism, the magnetism of 
volcanic formations, of mountain chains, of ferruginous beds ; 
some harder than others and therefore less subject to magnetic 
influence, but retaining its effects the longer ; some more affected 
by, the extremes of heat and cold, and hence exposed to more 
rapid and radical changes in their magnetic condition. 
But the question of the number of magnetic poles is leading 
us to another point of scarcely less importance, viz., the investi- 
gation of the changes that take place in the magnetism of the 
globe. The first point of inquiry is whether terrestrial 
magnetism as a whole is subject to continual change, and if so, 
are these changes periodical? Do they move in cycies? Do 
they follow any fixed laws that may lead to a knowledge of their 
causes ? 
The difficulty in answering these questions arises mainly from 
the irregular distribution of the points of maximum intensity ; 
but, granting that we meet with numerous exceptional cases, 
which no doubt will finally be discovered to depend on local in- 
fluences, we can trace a very regular and periodic change in all 
the magnetic elements. 
The first accurate observations that have come down to us are 
those of the declination, or variation of the compass, taken in 
Paris in 1541, when the needle pointed 8° to the east of the 
astronomical meridian. From that period the easternly devia- 
tions gradually increased, until it attained a maximum value of 
11° 30’ in 1580, when it returned slowly on its path and vanished 
in the year 1660, Paris being then on the curve of ‘‘no varia- 
tion.” Pursuing its westernly course, the needle pointed more 
and more west of north each year, and only reached its greatest 
western elongation of about 23° in 1814. The needle is at 
present returning towards the east, at the yearly rate of about 
9/5, and actuaily points rather less than 17° west of north. 
The variations of the declination at London have followed much 
the same order as those at Paris, nor has there been any great 
difference in the extent. 
The dip observations have unfortunately not been carried on 
so continuous!y during such a long series of years, and in conse- 
quence the secular variation of this element is less well deter- 
mined than that of the declination. With the exception of a 
single observation by Norman in 1576, who found the inclina- 
tion of the needle at London to be 71° 50’, we have scarcely any 
reliable data previous to 1720, when the dip had increased in 
London to 74° 42’. Since the latter epoch this element has 
always continued to decrease, being 70° 35’ in 1800, and now 
less than 68°, with an annval diminution of about 2''5. 
Of the secular variation of the intensity we know even less 
than of that of the dip, since the first observations date only as 
far back as the end of the last century ; and we have no less an 
authority than that of Sir Edward Sabine for the statement, that 
_ at commencement of the present century the bare fact of there 
being any difference whatsoever in the intensity of the magnetic 
force in different parts of the earth was unattested by a single 
173 
published observation.” The resuits, however, of modern re- 
search supply us with the important fact that the horizontal com- 
ponent of the intensity is at present rapidly increasing, its yearly 
rate of change being one 600th of its total value. 
Now each and all of these gradual variations in the several 
elements of the earth’s magnetism force upon us the conclusion 
that the magnetic pole must be endued with a motion of rotation 
in a more or less circular path around the pole of the earth’s 
axis. The results of such a rotation apparent to an observer 
situated for example in England will be easily understood if we 
consider for a moment the similar movement of any of the in- 
ferior planets in its orbit round the sun as viewed from the earth. 
Take Venus, for instance, which is the most conspicuous of the 
planets. At one time it may be seen moving away from the 
sun towards the east, when it is called the evening star, since i 
sets later than the sun. This outward movement continues for 
atime, until the planet reaches the point of its maximum elonga- 
tion; it then returns towards the sun, and after a time becomes lost 
to sight in the brilliancy of the solar rays, or on very rare occasions 
is visible in transit over the solar disc, as it will be for the first 
time this century in 1874, and again in 1882. Having passed 
the sun, Venus becomes the morning star, rising earlier and 
earlier until it has attained its greatest western elongation, when 
it again returns towards the sun. An analogous movement of 
the magnetic pole around the geographic pole has been clearly 
indicated by the secular variations of the declination, dip, and 
horizontal force. At the middle of the 16th century the bearings 
of the needle which would lead us to the magnetic pole were 
some 10° east of north. As time went on this deviation dimi- 
nished, whilst the dip increased, showing that the magnetic pole 
was approaching us, as it got nearer and nearer to the meridian, 
About the middle of the 17th century, or rather somewhat later, 
the magnetic pole crossed our meridian, which thus for the 
moment partly coincided with the “line of no variation.” From 
that time the needle has always pointed west, the western de- 
clination increasing more and more until the pole reached its 
maximum elongation in 1815. During this period thee was a 
gradual decrease in the dip, manifesting a recession of the pole, 
and this has continued steadily, though with diminished accele- 
ration, ever since the needle commenced its backward journey 
towards the geographic meridian. The present secular increase 
of the horizontal force also shows that the pole is receding, and 
that it will cross our meridian next on the further side of the geo- 
graphic pole. This will take place, according to the calculation 
of M. Quetelet, director of the Brussels Observatory, about the 
year 1940, and thus a complete revolution of the magnetic pole 
will occupy a period of some 560 years. Other physicists make 
this period longer. Local magnetism must of course interfere 
greatly with the position of the pole, and with its velocity of 
revolution, but this disturbing cause will affect still more the 
movements or form of the ‘‘ curve of no variation.” 
This rotation of the magnetic pole round the extremity of the 
earth’s axis bears so striking a resemblance to the motion of the 
pole of the heavens round the ecliptic, that we are led at once to 
inquire if anything can be detected in the magnetic rotation that 
corresponds with the inequalities in the precession of the earth’s 
axis, with the nutation caused by the action of the sun and moon. 
Are there, in other words, any annual, semi-annual, or monthly 
inequalities? The observations of the declination, taken during 
a series of years, and grouped together according to months, led 
to avariety of conclusions respecting the influence of the sun on 
the deflection of the needle. Arago agreed with Cassini in 
placing the sun jin the vernal equinox at the maximum western 
variation, and in the summer solstice at the minimum ; whilst 
Bowditch, in America, and Beaufoy, in England, both found 
that a maximum occurred in August and a minimum in Decem. 
ber, though a second maximum and minimum were placed by 
each in different seasons. The fact of some yearly range of the 
needle about its mean position appeared to be established ; but 
local influence seemed to have a Jarge share in determining the 
nature of the annual curve. 
(Ta be continued, ) 
SCIENTIFIC SERIALS 
THE Geological Magazine for December (No. 102) opens with 
a description by Mr. James Cartro of a new genus and species 
of fossil crustacea from the Upper Greensand of Lyme Regis, 
which the author proposes to name Orithopsis Bonneyi. The 
