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5 0 4 M A G N E T I C I N T E N S I T Y .
la id , a re im p o r ta n t w h e n e x trem e a c c u ra c y is s o u g h t; as for
ex am p le, in c om p a rin g th e fo rce a t s tations w h ich h a v e served
as th e u n itie s o f differen t o bservers : th e ir effect is, h ow ev er, o f
little m om e n t in observations w h ic h in c lu d e g re a t differences o f th e
te rre s tria l in te n s ity . B u t w h e n th e m a g n e tic co n d itio n o f th e
n e ed le h a s varied, a n d in te rp o la tio n becom es necessary, th e in stanc
e s a re r a re in w h ich i t can h e d one w ith e n tire satisfaction.
T h e tim e o f v ib ra tio n o f th is c y lin d e r a t P lym o u th , w ith c o rre c tions
a p p lie d fo r th e c h ro n om e te r's ra te , th e tem p e ra tu re , a n d th e
a rc , was, in D e cem b e r 1831, 7 7 0 ,8 seconds, w ith a d ip o f 69° 2 7',6,
a n d in O c to b er 1836, 7 7 7 ,3 seconds w ith a d ip o f 69° 17', 5.
T h e difference in its m a g n e tic state, a t th e tw o p e rio d s, is shown
h y th e sq u a re s o f th e tim e s o f v ib ra tio n m u ltip lie d b y th e cosine
o f th e d ip o b serv ed a t th e re sp e c tiv e periods. T h is m a k e s k n ow n
w h a t th e tim e o f v ib ra tio n o f th e same c y lin d e r w o id d h a v e b e en ,
h a d i t b e e n fr e e to move in th e d ire c tio n o f th e d ip p in g -n e e d le ,
in s te a d o f b e in g su sp en d e d h o riz o n ta lly ; an d , c o n seq u en tly , i f it
h a d b e en a c te d u p o n b y th e to ta l m ag n e tic in te n s ity , in s te ad o f b y
th e h o riz o n ta l com p o n en t o n ly. I t is h e re assumed th a t th e to ta l
te rre s tria l in te n s ity is c o n sta n t a t th e same pla c e. T h is is d o u b tle s s
n o t s tric tly tru e ; h u t th e am o u n t o f th e ch an g e m u s t h e too small
to re q u ire co n sid e ra tio n in th e p e rio d o c cu p ied b y C ap ta in F it z R
o y ’s o b s ervations. T h e h o riz o n ta l com p o n en t m u s t, h ow e v er,
ne ce ssa rily v a ry w ith th e ch an g e s in th e d ip ; a n d i t is, th e re fo re ,
from th e tim e o f v ib ra tio n in th e d ire c tio n o f th e d ip p in g -n e e d le ,
a n d n o t from th e tim e o f h o riz o n ta l v ib ra tio n , th a t th e ch an g e , or
oth e rw ise, in th e mag n e tism o f th e ne ed le is to b e in fe rre d . W e
find, th e n , th e e q u iv a len t tim e o f v ib ra tio n o f th is c y lin d e r in th e
d ire c tio n o f th e d ip p in g -n e e d le to h a v e b e en 45 6 ,4 seconds in
D e cem b e r 1831, a n d 4 6 2 ,2 s e c o n d s in O c to b e r 1836 ; sh ow in g a
difference o f 5 ,8 seconds in fifty -e ig h t m o n th s. I f w e compa re th is
am o u n t w ith th e fa r g re a te r loss o f In ten s ity su sta in e d b y th is
c y lin d e r in th e p re c ed in g vo y ag e, it seems a p ro b a b le su p p o sitio n
th a t, a t th e c om m en cem en t o f C a p ta in F itz -R o y ’s v oyage, th e cy lin d
e r h a d n e a r ly a tta in e d its p e rm a n e n t m a g n e tic state ; a n d th a t its
fu r th e r loss o f m ag n e tism , occasioning an in c re a se ot 5,8 seconds
in th e tim e o f v ib ra tio n , to o k p la c e in th e e a rly p a r t o f th e v oyage :
su p p o sin g th e loss to h av e b e en p ro g re s siv e , a n d n o t su d d en , as
from accidents, o f w h ic h th e o b serv atio n s g iv e n o in d ic a tio n s. Con-
M A G N E T I C I N T E N S I T Y . 5 0 5
s iste n tly w ith th is su p p o sitio n , th e loss h a s b e en d is tr ib u te d th ro u g h
th e first h a lf, o r tw e n ty -n in e m o n th s , o f th is vo y ag e, in th e p ro p o r tio
n o f th re e -te n th s o f a second p e r m o n th in th e first te n m o n th s ,
com m en cin g D e c em b e r 1st 1831 ; tw o -te n th s p e r m o n th in th e
n e x t n in e m o n th s ; a n d o n e -te n th p e r m o n th in th e rem a in in g n in e
m o n th s . In th e la s t tw e n ty -n in e m o n th s o f th e v o y ag e, th e
in te n s ity o f th e c y lin d e r is su p p o se d to h a v e b e e n u n ifo rm , a n d
th e same w h ic h i t was fo u n d to possess on th e r e tu r n to E n g la n d
in 1836.
I t is s atisfa c to ry th a t, w ith th is compensation, th e observations
a t P o r t P r a y a , in J a n u a ry 1832, a n d in S e p tem b e r 1836, assign
almost id e n tic a lly th e same re la tiv e m ag n e tic in te n s ity to th a t
station.
T h e corre c tio n fo r tem p e ra tu re fo r th is c y lin d e r n o t h a v in g
b e en p re v io u s ly e x am in ed , I re c e iv ed i t from C ap ta in F itz -R o y
for th a t p u rp o s e , a n d m ad e w ith it th e fo llow in g o b s ervations.
T h e c y lin d e r, in its ow n a p p a ra tu s , was p la c ed in a la rg e e a rth e n
ja r , g la z ed a t th e to p , a n d s tan d in g in a la rg e r e a rth e n vessel, in to
w h ic h w a rm w'ater co u ld b e p o u re d a t p le a su re , a n d th e c y lin d e r
wa s th e n v ib ra te d a lte rn a te ly in h e a te d a ir a n d in a ir o f th e n a tu ra l
tem p e ra tu re . T h e s e e x p e rim e n ts w e re m ad e a t T o r tin g to n , in
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