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t t o i f i ;
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lit
N ' l i , , l i ! ! ! * T 'M
T h e o b je c t o f n o tin g th e tim e o f e v e ry te n th v ib ra tio n is to
ch eck e rro r s in th e c o u n tin g , w h ic h w ill sometimes o c cu r in th e
c o u rse o f th e 360, p a rtic u la rly w ith th e v e ry s h o rt a n d q u ic k -
m o v in g ne ed les o f M. H a n s te e n 's v e ry p o rta b le a p p a r a tu s , a n d a t
s tations o f low d ip , w h e re th e h o riz o n ta l fo rc e is g r e a te s t, a n d th e
n e ed le co n se q u e n tly moves m o s t q u ic k ly . Sev e ra l su ch m is ta k e s
e v id e n tly o c cu rred . W h e n th e tim e o f c om p le tin g every su p p o se d
te n th v ib ra tio n is o b s erv ed w ith to le ra b le exactness, a n d th e d u r a tio
n o f e ach p a ir o f v ib ra tio n s d e c id e d ly exceeds any ir r e g u la r ity o f
p ro b a b le o c cu rren c e , a p a r t from m is c o u n tin g th e n um b e r o f v ib ra tio
n s, su ch m is tak e s can b e d isco v e red w ith ease, a n d re c tifie d
w ith c e rta in ty . T h is h a s b e en d o n e in e v e ry case w h e re no d o u b t
c o u ld e x ist o f a m is tak e o f th e k in d h a v in g o c cu rre d ; su c h as
w h e n a ll th e in te rv a ls ai-e o f n e a rly e q u a l d u ra tio n , w ith one o r
tw o e x cep tio n s, w h ich d iifer as m u c h as th r e e o r fo u r seconds from
th e g e n e ra l b o d y . T h e re a re tw o s tations, h ow e v e r, C allao a n d
K e e lin g Islan d s, w h e re th e re c tific a tio n is n o t so c le a r, o r th e tru e
r e s u lt so o b vious. A t Callao th e r e a re th re e series o f h o riz o n ta l in te
n sitie s , e ach o f fo r ty o b s e rv ed in te rv a ls , w h ic h sh o u ld b e o f te n
v ib ra tio n s each. S e v e ra l o f tliese in te rv a ls a re b e tw e e n 17,5 a n d
18,5 seconds, a n d s ev e ra l o th e rs b e tw e e n 2 0 ,5 a n d 21,5 seconds.
T h e se can h a rd ly re p re s e n t a n e q u a l n um b e r o f v ib ra tio n s , be cau se
th e difference b e tw e e n th em is g re a te r th a n c an e asily b e su p p o sed
d u e to a n y u n c e r ta in ty in se iz in g th e p a rtic u la r b e a t o f th e c h ro n
om e te r a t w h ic h th e v ib ra tio n was c om p le ted ; it is, mo reo v e r,
a b o u t th e tim e th a t w o u ld b e o c cu p ied h y tw o v ib ra tio n s m o re or
less. T h e q u e stio n th e n arises, do th e lo n g e r in te rv a ls re p re s e n t
1 2 , a n d th e sh o rte r 10 vib ra tio n s , or d o th e lo n g e r re p re s e n t 10 ,
a n d th e sh o rte r 8 ? In th e fo rm e r su p p o sitio n th e in te n s ity a t
Callao w o u ld b e a b o u t 1 .0 1 (P a r is= 1 .3 4 8 ) : in th e la tte r ab o u t
0 .7 5 . T h e difference shew s h ow g r e a t an e rro r w o u ld h e r is k e d h y
e ith e r a ssum p tio n . I f w e ta k e a m e a n o f a ll th e in te rv a ls as th e y
s tan d , th e am o u n t o f e rro r r is k e d w o u ld be c e rta in ly le ss en ed ;
b u t w e sh o u ld a s su re d ly n o t h a v e th e t r u e tim e o f th re e h u n d r e d
v ib ra tio n s , e x c e p t on one su p p o sitio n : n am e ly , th a t th e ir r e g u la
ritie s in q u e stio n a re n o t e rro rs in e stim a tin g th e n um b e r o f
vib ra tio n s , b u t th a t e ach in te rv a l re a lly re p re s en te d a n e q u a l
n um b e r, a n d th a t some u n u su a l a n d a cc id en ta l cause occasioned
th e n e ed le to differ so g re a tly in successive in te rv a ls . B u t th is
su p p o sitio n w o u ld im p ly a d is tu rb in g cause v itia tin g th e series as
a m e a su re o f th e m ag n e tic in te n s ity a t th e s ta tio n . I h a v e n o t v en tu
re d , th e re fo re , to d raw a n y conclusion from th e se o b s ervations,
f a r th e r th a n to n o tic e , as above, th e lim its w ith in w h ic h , in e ith e r
o f th e tw o first su p p o sitio n s, th e in te n s ity w o u ld fall.
A n e a rly s im ila r re a so n in g ap p lie s to th e o b s erv atio n s a t K e e lin
g Is lan d s ; o f th r e e series, one is d e c id ed ly so irr e g u la r , th a t no
infe re n c e c o u ld b e d raw n from it ; in th e tw o o th e r series th e i r r e g
u la ritie s a re n e ith e r so fre q u e n t, n o r so la rg e : m y g e n e ra l im p r e s sion
(in th e u n c e r ta in ty c re a te d b y th e ir r e g u la r ity o f th e first
series), is, th a t th e m a jo rity o f th e in te rv a ls a re o f tw e lv e v ib ra tio
n s , a n d n o t o f te n ; i f o f tw e lv e , th e in te n s ity w o u ld h e a b o u t
1,21 ; i f o f te n , a b o u t 0,85.
T h e in co n v en ien c e o f th e r a p id m otion o f th e n e ed le, occasioned,
a t one p a r t o f th e vo y ag e, th e p ra c tic e to h e d is co n tin u e d o f
o b s e rv in g e v e ry te n th v ib ra tio n , a n d e v e ry tw e n tie th was s u b s titu
te d . T h is no d o u b t re lie v ed th e p e rp le x ity in w h ic h th e o b s e rv e r
occasionally fo u n d h im se lf, in h a v in g to o b serve, a n d re c o rd , an d
b e p r e p a re d ag a in to observe, a t ev e ry tw e n ty seconds o r less, a n d
so fa r th e ch an g e e n ab led liim to observe b e tte r . B u t s till, th e
d is ad v a n ta g e rem a in s , in so q u ic k m o v in g a n e ed le , th a t i f a m is ta
k e o f tw o v ib ra tio n s is made , th e d ifferen c e o f tim e occasioned
is n o t o f so m a rk e d a n d d e cid ed a c h a ra c te r as to b e a t all times
a t once d is tin g u ish ed . I t is o f m u c h m o re im p o rtan c e th a t th e r e
sh o u ld b e no m is c o u n t o f th e v ib ra tio n s , th a n th a t th e tim e s sh o u ld
b e re c o rd e d c o rre c tly to th e fra c tio n o f a second. I t is o n ly th e
e a rlie r a n d la te r tim e s th a t a re finally in flu en tia l ; b u t e v e ry u n d e te
c te d e rro r in th e n um b e r o f v ib ra tio n s falls w ith its w hole w e ig h t
u p o n th e re su lt.
T h e occasional discrepancies in th e re s u lts o f th e same, o r o f
different, observers, or on th e same, o r on different, day s, w h ic h
a re seen in th e su b jo in ed ta b le , a re n o t, I b e liev e , tra c e ab le to th e
so u rc e I h a v e h e e n d iscu s s in g , nor a p p a r e n tly to a n y o th e r th a n an
a c tu a l difference in th e tim e o f th e c y lin d e r p e rfo rm in g its v ib ra tio n .
A m e a n h a s b e en ta k e n as th e re s u lt a t e ach s ta tio n , e x c e p t a t S t.
H e len a , w h e re th e d is c rep an cy on th e 1 1 th a n d 1 3 th o f J u ly wa s so
co n sid e rab le , th a t i t h a s b e en th o u g h t m o re s a tisfa c to ry to collect
th e o b serv atio n s o f each d ay in to sep a ra te re su lts .
T h e su b jo in e d ta b le comprises th e r e s u lt o f each ob serv atio n .
' lA
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