! ' l
■■ V
. '(
A t R io de Ja n e iro , w h ic h was th e first sta tio n o b s erv ed a t in
S o u th Am e ric a , th e c y lin d e r was v ib ra te d in A u g u s t 1826, S e p tem
b e r 1827, a n d D e c em b e r 1828 ; in th e in te rv a ls b e tw e e n th e se
da te s a re com p rise d th e g re a te r p a r t o f th e o b serv atio n s on th e
e a s t side o f S o u th Am e ric a . T h e r e is n o d ire c t observ atio n a t R io
su b se q u e n tiy to D e c em b e r 1828 ; b u t w e a re e n ab led to s u p p ly th e
tim e o f v ib ra tio n , w h ic h w o u ld h a v e b e en ob serv ed h a d th e c y lin d
e r b e e n em p lo y ed a t R io on J u n e 1, 1830, in th e fo llowin g m a n n
e r. W e h a v e seen th a t on th e 1 5 th S e p tem b e r 1827 th e tim e o f
v ib ra tio n w a s o b s erv ed a t R io ; on th e 1 8 th D e c em b e r fo llow in g it
wa s o b s erv ed a t M o n te V id e o . T h e s e o b serv atio n s g iv e th e in te n s
ity a t M o n te V id e o re la tiv e ly to th a t a t R io , su b je c t to w h a te v e r
ch an g e o f m a g n e tism th e c y lin d e r m ay h ave u n d e rg o n e in th e
in te rv a l o f th re e m o n th s . T h is comparison was re p e a te d in th e
fo llowin g y e ar, on C ap ta in K in g ’s r e tu rn from M o n te V id eo to
R io , th e in te rv a l b e in g n e a r ly o f th e same d u ra tio n , a n d th e o rd e r
o f th e e x p e rim e n t re v e rs e d , th e pa ssag e b e in g in th is in s ta n c e from
M o n te Video to R io , i t h a v in g h e e n b e fo re from R io to M o n te
Video. O n th e su p p o sitio n o f a n u n ifo rm , o r n e a r ly u n ifo rm ra te
o f c h an g e in th e c y lin d e r, th e e rro rs a ris in g th e re from d u rin g th e
tw o p a ss ag es w o u ld b e o f o p p o site k in d s , a n d sh o u ld compensate
e ac h o th e r in a m e a n o f th e tw o comparisons. C allin g th e fo rce a t
Rio u n ity , th e se comparisons g iv e its v a lu e a t M o n te Vid eo re sp ec tiv
e ly as follows, n am e ly .
S e p tem b e r a n d D e c em b e r 1827
O c to b e r a n d D e c em b e r 1828
1 -1 9 7 /
1 -2 0 7 .
Mean 1-202.
O n th e 1st o f J u n e 1830, b e in g th e n on h is r e tu r n from th e w e st
coast o f S o u th America, a n d on th e eve o f sailin g fo r E n g lan d ,
C ap ta in K in g ag ain o b s erv ed th e tim e o f v ib ra tio n o f th e c y lin d e r
a t M o n te V id e o ; w h e n c e , th ro u g h th e p re c e d in g comparison, w e
o b ta in th e tim e o f v ib ra tio n a t R io , w h ich sh o u ld b e lo n g to th e
same da te . W e h a v e th u s a fo u r th d a te a t R io , w h ic h , a d d e d to
th o se e n um e ra ted above, w ill in c lu d e th e w h o le o f th e S o u th Am e ric
a n s tations ; a n d w e h a v e o n ly to d is trib u te in each in te rv a l th e
loss o f m a g n e tism w h ic h th e o b serv atio n s sh ew to h a v e ta k e n p la c e
from one d a te to th e n e x t, in th e m a n n e r w h ic h m a y a p p e a r m ost
su ita b le . T h e re is no v e ry obvious in d ic a tio n th a t th e loss was
o th e r th a n g ra d u a l ; a n d b y co n sid e rin g it u n ifo rm in e ach sep a ra te
in te rv a l, th e re su lts a re fo u n d e x trem e ly a c c o rd a n t a t seve ra l o th e r
statio n s a t w h ich observ atio n s w e re r e p e a te d a t d is ta n t in te rv a ls .
I n th e first o f th e su b jo in e d tab le s a re g iv en th e tim e s o f v ib r a tio
n a t R io a t th e fo u r p e rio d s re fe rr e d t o ; a n d th e c o rre sp o n d in g
tim e s as a d ip p in g n e ed le. I n its th r e e la s t co lum n s a re sh ew n ,—
th e n um b e r o f d ay s c om p ris e d in e ach in te rv a l,— th e in c re a se in th e
tim e o f v ib ra tio n ow in g to th e loss o f m a g n e tism ,— a n d th e re s u ltin
g d a ily co rre c tio n , on th e su p p o sitio n in e ach case o f th e loss
h a v in g b e e n u n ifo rm in th e in te rv a l d u r in g w h ic h i t o c cu rre d .
T h e second ta b le c o n ta in s th e c o rre c te d tim e s o f h o riz o n ta l
v ib ra tio n a t e a c h o f th e S o u th A m e ric an s tatio n s, a t th e d a te s
re sp e c tiv e ly in s e rte d , — th e c o rre sp o n d in g tim e s as a d ip p in g
n e ed le,— th e tim e s o f v ib ra tio n as a d ip p in g n e ed le a t R io de
J a n e iro a t th e same d a te s, d e riv e d from th e o b serv atio n s in th e
first ta b le ,— a n d th e r e s u ltin g in te n s ity a t e a ch station re la tiv e ly to
u n ity a t Rio. T h u s fa r th e r e s u lts a re d e riv e d from C ap ta in K in g ’s
o b s ervations, u nm ix e d w ith th o s e o f a n y o th e r o b s e rv e r : b u t in
o rd e r to b r in g C ap ta in K in g ’s series in to co nnexion w ith th e g e n e r
a l b o d y o f re su lts o f o th e r o b s e rv e rs, th e v a lu e s o f h is in ten s itie s
a re e x p re ss ed in th e final c o lum n in te rm s o f th e scale in common
u se , in w h ic h th e fo rce a t P a r is = 1 ,348, a n d a t R io de J a n e iro
0 ,8 8 4 ; th e la tte r b e in g th e m e an o f fo u r in d e p e n d e n t d e te rm in
a tio n s b y th e fo llowin g ob serv ers, n am e ly .
1817 a n d 1820 F r e y c in e t .................. 0,890
1 8 2 7 ................... L iitk e ........................ 0,886
183 0 ................... E rm a n ......................... 0 ,8 7 9 j
1 83 6 ................... F i t z - R o y ................... 0 ,8 7 8 J
0 ,884
P o r t S t. E len a is n o t in c lu d e d in th is ta b le , as n o d ip was ob serv ed
th e re a n d th e to ta l in te n s ity c o n s e q u e n tly c an n o t b e com p u ted .
T h e th re e s tations, M ad e ira , Teneriffe, a n d P o r t P r a y a , a t w h ic h
th e c y lin d e r wa s v ib ra te d in th e o u tw a rd vo y ag e, a re also w ith o u t
d ip s o b serv ed b y C ap tain K in g . T h e deficiency a t P o r t P r a y a h a s
b e en su p p lie d from C ap ta in F itz -R o y ’s o b serv atio n s a n d m y own,
b o th h a v in g b e en m ad e a t th e same p la c e a t w h ic h C ap ta in K in g ’s
in te n s ity was o b s e r v e d ,-C a p t a in F it z -R o y ’s a t a la te r, a n d m in e a t
a n e arlie r d a te . A t M ad e ira also th e d ip h a s b e en su p p lie d from
my observations, w h ic h w e re m ad e in th e B r itis h co n su l’s g a rd e n
a t F u n c h a l, w h e re C a p ta in K in g ’s c y lin d e r was v ib ra te d . I h av e
lm