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2. Observalions o f Inte n s ity .
T h e m e th o d em p lo y e d b y C ap ta in F itz -R o y to d e te rm in e th e
v a ria tio n s o f th e m a g n e tic fo rce was th a t o f n o tin g th e tim e o f
v ib ra tio n o f a m a g n e tic c y lin d e r su sp en d e d h o riz o n ta lly .
T h e c y lin d e r wa s on e w h ic h h a d b e en g iv e n h y M. H a n s te e n ,
in 1826, to C ap ta in P h illip P a r k e r K in g , R .N ., a n d h a d b e en u sed
b y h im d u rin g th e s u rv e y o f th e coast o f S o u th Am e ric a , w h ic h h e
c o n d u c ted from 1826 to 1830. T h e a p p a r a tu s in w h ic h it was
v ib ra te d , b o th in C ap ta in K in g ’s a n d C ap ta in F itz -R o y ’s v o y ag es ,
wa s th e w e ll-k n ow n one o f M . H a n ste e n .
B y o b serv atio n s m a d e w ith th is c y lin d e r on th e 2 2 d M a rch
1826, a n d a g a in on th e 2 4 th J a n u a ry 1830, in th e g a rd e n o f th e
R o y a l O b se rv a to ry a t G re e nw ich , i t a p p e a re d th a t its tim e ot
p e rfo rm in g 30 0 v ib ra tio n s h a d in c re a sed from 734,45 seconds in
1826, to 7 7 5 ,8 0 seconds in 1 8 3 1 ; o r 4 1 ,3 5 seconds in 1,770 days.
A c h an g e o f su ch m a g n itu d e in th e m a g n e tic in te n s ity o f th e
in s trum e n t em p lo y ed to m e a su re th e v a ria tio n s o f th e te rre s tria l
in te n s ity , a n d w h ich o u g h t itself, th e re fo re , to b e in v a riab le , w o u ld ,
in th e g e n e ra lity o f cases, h a v e p r e v e n te d a n y satisfa c to ry co n c lu sion
w h a tso e v e r b e in g d raw n from th e o b servations. F o r tu n a te ly
from th e n a tu r e o f th e d u tie s in w h ic h C ap tain K in g was
en g ag e d , h e h a d occasion to r e tu r n fre q u e n tly to th e same
an ch o rag e s , a n d b y h is e x trem e care to re p e a t observ atio n s on e v e ry
su ch r e tu rn , he h a s p ro v id e d a me an s o f c om p u tin g th e decrease o f
th e In te n s ity o f th e c y lin d e r, p ro p o rtio n e d to in te rv a ls o f tim e ,
b e tw e e n 1826 a n d 1831 ; a n d o f th u s in tro d u c in g compensations
fo r it, w h ic h re n d e r th e re su lts on th e wh o le n e a r ly as s atisfa c to ry
as i f th e c y lin d e r h a d p re s e rv e d a n u n ifo rm m a g n e tic condition
th ro u g h o u t.
T h e v o y a g e w h ich C ap ta in F itz -R o y h a d to p e rfo rm p rom ised
to fu rn ish few , i f a n y , su c h o p p o rtu n itie s o f e x am in in g th e state
o f th e m a g n e tism o f th e c y lin d e r, b e tw een th e d e p a r tu re from a n d
th e r e tu rn to E n g l a n d ; a n d ,— as i t c an n o t b u t b e ex trem e ly dis-
c o u ra g in g to officers to m a k e o b serv atio n s w h ic h th e y h a v e reason
to a p p re h e n d m a y p ro v e u n a v a ilin g from d e fe c t in th e in s trum e n t
em p lo y e d ,—it m u s t b e r e g a rd e d as ex c e e d in g ly c re d ita b le to
C ap ta in F itz -R o y a n d h is officers, th a t, w ith th e k n ow le d g e o f the
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ch an g e w h ich th e c y lin d e r h a d u n d e rg o n e in th e p re c ed in g
voyage, th e y p e rs ev e re d in d ilig e n tly o b s erv in g , a n d c arefu lly
re c o rd in g , its tim e o f v ib ra tio n , a t most o f th e p rin c ip a l p o rts w h ich
th e y v is ited in th e ir voyage o f five y e a r s ’ d u ra tio n . N o r was it
u n til th e ir r e tu rn to th e C ap e V e rd Isla n d s, in S ep tem b e r 1836,
th a t th e y could in fe r, from o b serv atio n s re p e a te d a t th e same sp o t
as in th e ir o u tw a rd passage in 1832, th a t th e c y lin d e r h a d n o t
v a rie d in an y th in g lik e th e d e g re e th a t i t h a d d o n e in th e p re c e d
in g v o y a g e , an d th a t th e c are a n d p a in s th e y h a d b e s tow e d we re
th e re fo re lik e ly to b e re com p e n se d b y success.
T h is a p p e a rs a fittin g o p p o rtu n ity to r em a rk , h ow m u c h th e
esta b lishm en t in E n g la n d o f a d ep o sito ry fo r m ag n e tic n e ed les is
n e ed ed ; w h en ce officers, a n d p e rsons d e siro u s o f m a k in g such
ob serv atio n s, m ig h t b e su p p lie d w ith in s trum e n ts , w h ic h h a d b e en
k e p t a sufficient tim e to h a v e a tta in ed th e ir p e rm a n e n t m a g n e tic
sta te , a n d h a d b e en ex am in ed from tim e to tim e to p ro v e th a t th e y
h a d d one so. T h e c o rre c tio n fo r tem p e ra tu re sh o u ld b e a sc e rta in ed
fo r each n e ed le , a n d g iv en w ith i t ; as w e ll as th e tim e o f v ib ra tio n
(o r w h a te v e r else c o n s titu te d th e m e a su re o f in te n s ity ,— as, fo r
ex am p le , th e an g le o f deflection in M r. L lo y d ’s statica l ne ed les,)—
o b s e rv ed a t th e sp o t w h ic h sh o u ld h e s ele c ted as m o s t su itab le fo r
a p o in t o f g e n e ra l c om p a riso n ; a n d th e o b serv atio n s sh o u ld be
re p e a ted a t th e same sp o t on th e r e tu rn o f th e n e ed le. T h e w a n t o f
s u c h a n e sta b lishm en t h a s lo n g b e en g re a tly f e l t ; a n d o p p o rtu n itie s ,
w h e re n o th in g wa s w a n tin g b u t p ro p e r in s trum e n ts , h a v e b e e n lo s t
in co nsequence, w h e re d e te rm in a tio n s o f g r e a t v a lu e m ig h t h av e
be en ob ta in ed , in p a rts o f th e w o rld o f th e h ig h e s t m a g n e tic in te re s t,
a n d w h e re su ch o p p o rtu n itie s a re o f r a re o c cu rren ce .
T h e corre c tio n s n e ce ssa ry to r e n d e r th e tim e s o f v ib ra tio n a t th e
differen t s tations s tric tly c om p a rab le w ith e ach o th e r, a re as
follows.
1st, F o r th e ra te o f th e c h ro n om e te r.
2 d , F o r th e tem p e ra tu re o f th e n e edle.
3 d , F o r th e a rc o f v ib ra tio n .
4 th , F o r a n y change in th e m a g n e tic c o n d itio n o f th e cy lin d e r.
I n ex te n siv e v o y ag e s , th e la s t-n am e d co rre c tio n , o r th a t fo r th e
c h a n g e in th e c y lin d e r itself, is th e one w h ic h re q u ire s p rin c ip a l
conside ra tion. T h e corre c tio n s fo r tem p e ra tu re , a n d for th e arc ,
on th e first o f w h ich p a rtic u la rly m u c h s tre ss h a s sometimes b e en
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