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O t a h e i t e .
Observer. Date. Variation,
Dook
W a l e s . .
B a y l e y . .
V a n c o u v e r
D u p e r r e y
F i t z - R o y . .
1 7 6 9
1773
1774
1794
1 8 2 3
1835
4 . 45 E .
5. 40 E.
5 . 49 E .
6 . 1 2 E .
6 . 4 0 E .
7 . 34 E.
B a y o f I s l a n d s , N. Z e a l a n d .
8 . 4 0 E ,
1 1 . 2 5 E .
1 3 . 2 2 E .
1 4 . 0 0 E .
T a s m a n .
C o o k
D u p e r r e y
F i t z - R o y .
C o o k
H u n t e r .
. . 1 6 4 3
. . 1 7 6 9
. . 1 8 2 4
. .. 1 8 3 5
S y d n e y .
, . . 1 7 7 0
. . . 1 7 8 7
0 0 E .
3 0 E .
Date. Variation.
Flinders ..
Freycinet..
Duperrey..
Fitz-Roy ..
1 8 0 3
1 8 1 9
1 8 2 4
1 8 3 6
8 . 5 1 E .
9 - 1 5 E .
8 . 5 6 E .
1 0 . 2 4 E .
V a n D i e m e n ’s L a n d .
T a s m a n . . . . 1 6 4 2 3 . 0 0 E .
B a y l e y . . . . 1 7 7 7 7 . 2 9 E .
B i i g h . . . . 1 7 7 8 8 . 3 3 E .
F i t z - R o y . . . . 1 8 3 6 1 1 . 0 6 E .
M a u r i t i u s .
K e e l i n g . . . . 1 6 0 9 2 1 . o W .
M a t h e w s . . . . 1 7 2 2 1 9 . 4 W .
F r e y c i n e t . . 1 8 1 8 1 2 . 4 6 W .
D u p e r r e y . . 1 8 2 4 1 3 . 4 6 W .
F i t z - R o y . . . . 1 8 3 6 u . 1 8W .
W e m a y d e riv e from th e facts in th e above ta b le th e fo llowing
g e n e ra l a n d e asily rem em b e re d conclusion in r e g a rd to th e ch an g e s
o f th e v a ria tio n in th e s o u th e rn h em is p h e r e ; n am e ly , th a t ta k in g for
o u r p o in t o f d e p a r tu re th e m e rid ia n o f 65° w e st in S o u th Am e ric a ,
■we find th a t a t all th e statio n s e ast o f th a t m e rid ia n to th e C ap e o f
G ood H o p e in c lu s iv e , th e n o r th p o le o f th e n e ed le has moved
tow a rd s th e w e s t ; a n d th a t a ll th e s tations w e s t o f th e same m e rid
ia n to M a u r itiu s in c lu siv e, th e n o rth pole of th e n e ed le h a s moved
tow a rd s th e east.
A n a lm o st e q u a lly simple g e n e ra liz a tio n m ay b e d raw n in re sp e c t
to th e ch an g e s o f s itu a tio n o f th e lin e s o f e q u a l v a ria tio n in th e
so u th e rn h em isp h e re : b u t h e re it m u s t b e su p p o se d e ith e r th a t th e
r e a d e r is th o ro u g h ly fam ilia r w ith th e g e n e ra l a rra n g em e n t o f th e se
lin e s , o r th a t h e h a s a m a p o f th em b e fo re h im . H e w ill fin d such
a m a p o f tb e v a ria tio n lin e s, in 1787 in th e F if th R e p o r t o f th e
B ritis h A ssociation ; b u t a n y o th e r m ap, c o rre sp o n d in g to a n y epoch
w ith in th e la s t h u n d r e d y e ars, w ill e q u a lly serve th e p u rp o s e .
R e fe rrin g to su ch a m a p , it w ill be seen th a t th e lin e s o f v a ria tio n
in th e S o u th Pacific, fo rm a sy stem o f n e a rly co n c e n tric cu rv e s, o f
a n oval, or p e a r-sh ap e d fo rm , th e o u ts id e cu rv e s h a v in g a h ig h e r
v a ria tio n , w h ic h p ro g res siv ely d im in ish e s to th e c en tre . W e may
re g a rd th is sy stem a s c om p re h e n d in g th e wh o le o f th e g eo g ra p h ic a l
space b e tw e e n th e co ast o f S o u th Am e ric a a n d th e m e rid ia n o f
N ew Z e a la n d . T h ro u g h o u t th is space th e v a ria tio n is e a ste rly , a n d
in c r e a s e s : we m ay co n seq u e n tly c h a ra c te riz e th e c h an g e in th e
s itu a tio n o f th e lines o f e q u a l v a ria tio n as a p ro g res siv e clo sin g -in
o f th e cu rv e s from a ll sides tow a rd s th e c en tre , b y w h ich th e a re as
sev e ra lly c om p reh en d ed b y th em become less ; a n d th e low e r v a ria tio
n s, as th e y successively r e a c h th e c en tre , d is ap p e a r, a n d a re
re p la c e d b y th e c lo sin g -in o f th o se o f n e x t h ig h e r am o u n t. T h e
c h an g e s w h ich h a v e ta k e n p la c e a t a ll C ap ta in F it z -R o y ’s statio n s
com p ris ed w ith in th e space re fe rr e d to , a re a c c o rd a n t w ith th e
sy stem a tic a lte ra tio n th u s d e sc rib ed .
I n all o th e r p a rts o f th e h em isp h e re th e lin e s o f v a ria tio n h ave
a p ro g re s siv e w e ste rly m o v em en t, a n d to th is also C ap ta in F it z R
o y ’s observ atio n s co rre sp o n d .
I t follows, from w h a t h a s b e e n s ta ted , th a t th e lines on th e
w e s te rn sid e o f th e co n c en tric sy stem in th e S o u th Pa c ific h a v e a n
e a s tw a rd m o v em en t, w h ic h p re s e n ts a n a p p a r e n t a n om a ly to th e
g e n e ra l p ro g re s s o f th e lin e s o f v a ria tio n in th e s o u th e rn h em is p
h e re , w h ic h is from e a s t to west.
O ta h e ite , a n d th e B a y o f Is lan d s in N ew Z e a lan d , p re s e n t e x am p
le s o f ch an g e s in th e v a ria tio n c o rre sp o n d in g to th is a p p a r e n t
anomaly. T h e consistency, h ow ev e r, b o th o f th e m o v em en t a n d o f
th e c o n fig u ra tio n o f th e lin e s o f v a ria tio n in th is q u a rte r, w ith
th o s e in o th e r p a r ts o f th e so u th e rn h em isp h e re , a n d w ith th e
g e n e ra l sy stem o f th e m ag n e tic p h e n om en a, h a s b e en sh ew n b y
M r. H a n s te e n in th e M ag n e tism u s d e r E rd e , an d in th e A n n a len
d e r P h y s ik , vol. x x i.
T h e a n n u a l am o u n t o f th e ch an g e o f th e v a ria tio n a p p e a rs cons
id e rab ly g re a te r a t th e Cape o f G ood H o p e an d M a u r itiu s th a n
a t a n y o th e r o f C a p ta in F itz -R o y ’s s tations, am o u n tin g to a b o u t
e ig h t m in u te s ; sh ew in g th a t th e v a ria tio n lin e s in th a t q u a r te r a re
c h a n g in g th e ir p o sitio n more r a p id ly th a n e lsew h e re . T h e n o rth
p o le o f th e n e ed le is m o v in g to th e w e s t a t th e C ape, a n d to th e
e a s t a t M a u r itiu s ; b u t i t w ill b e seen, b y a re fe re n c e to th e m ap,
th a t th e se opposite m o v em en ts a re in p e rfe c t c o rre sp o n d en c e
w ith th e u n ifo rm w e ste rly p ro g re s sio n o f th e va ria tio n lines, a n d
r e s u lt from th e ir configuration.
A t th e statio n s in th e v ic in ity o f th e m e rid ia n o f 6 5 “ w e st, th e
ch an g e a p p e a rs to b e v e ry small.
[T h e variation at Ascension (13° 3 0 'W.) is correctly inserted in the
tables: it is the only one of Captain Fitz-Ro y ’s stations at which his
observations are not accordant with those of other observers : the discordance
may be occasioned by the great prevalence o f local disturbances
at Ascension.]
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