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W h en c e th e fo rm u la T '= T [1 x 0-000068 (6 0 °—t ) ] , in -w h ic h
T is th e tim e o f v ib ra tio n a t an y s tation, t, th e tem p e ra tu re o f th e
c y lin d e r in d eg re es o f F a h re n h e it, a n d T ' is th e e q u iv a len t tim e
a t a s ta n d a rd tem p e ra tu re o f 60°. T h e th e rm om e te r was n o ted
a t th e b e g in n in g a n d e n d in g o f e v e ry s et o f v ib ra tio n s , an d was
a lw a y s p la c ed In th e b o x w ith th e c y lin d e r.
A t all C ap ta in F it z -R o y ’s s tations th e a p p a r a tu s was p la c ed for
o b serv atio n o n a s tan d , w h ic h ra is ed i t from tw o to th r e e fe e t
above th e g ro u n d , th e re b y re n d e r in g th e c y lin d e r som ew h a t less
liab le to b e d is tu rb e d b y loc a l in flu en c e s: i t was n o t fu rn ish e d w ith
a me ans o f e x am in in g th e s tric t h o riz o n ta lity o f th e c y lin d e r, th a t
im p ro v em e n t h a v in g b e en in tro d u c e d in to M . H a n s te e n ’s a p p a ra
tu s a t a la te r p e rio d . O n th is p o in t C ap ta in F it z -R o y rem a rk s :
“ A small le a d en trip o d was u s ed as a s tan d , whose u p p e r surface
“ was a d ju s te d b y a small sp irit-lev e l— or ro u g h ly b y th e tro u g h
“ o f a n a rtificia l h o rizo n , filled w ith m e rc u ry . U p o n th e le a d en
“ s ta n d th e b o x co n ta in in g th e n e ed le w a s a d ju s te d h y its foot-
“ screws, so th a t th e s u sp e n d in g fibre o f s ilk h u n g c e n tra lly in
“ th e wo o d en tu b e , th e n e ed le ’s c e n tre b e in g o v e r th a t o f th e
“ g ra d u a te d circle, a n d th e n e ed le its e lf n e a r, b u t n o t to u c h in g ,
“ th e b o ttom o f th e b o x . T h e n e ed le was n o t a lways s tric tly
“ p a ra lle l to th e b o ttom o f th e b o x , n o r s tric tly h o riz o n ta l, be-
“ cause I w o u ld n o t move th e b ra s s s tir ru p in w h ic h it was su s-
“ p e n d ed , b u t its d e v ia tio n from s tric t h o riz o n ta lity never ex ce e d e d
“ tw o d eg re es, a n d was seldom n e a rly so m u c h .”
T h e tim e o f c om p le tin g ev e ry te n th v ib ra tio n was re c o rd e d .
T h e tim e o f p e rfo rm in g 3 0 0 v ib ra tio n s is d e d u c e d from a m e an
g e n e ra lly o f seven p a rtia l re su lts ; i. e. from th e Oth to th e 3 0 0 th J
1 0 th to th e 3 1 0 th ; a n d so on, to th e 6 0 th a n d 3 6 0 th v ib ra tio n s ;
th e c om m en cin g v ib ra tio n b e in g a lw ay s a t a n a rc o f 2 0°. I n a v e ry
few in s tan ce s th e n um b e r o f v ib ra tio n s o b s e rv ed , a fte r th e comm
en c in g a rc o f 20°, was less th a n 3 6 0 ; in su ch cases th e first v ib r a tio
n a fte r th e a rc h a d become 20° h a s still b e en ta k e n as th e comm
e n c in g one, th o u g h p re v io u s ones m a y h ave b e en re c o rd e d ; it
b e in g k e p t s tric tly in view, to o b ta in th e re la tiv e tim e o f v ib ra tio n
in a rc s as n e a rly th e same as possible, a n d n o t ex c e ed in g 20° as th e
in itia l. T h e a rc w a s re d u c e d to 10° g e n e r a lly a b o u t th e 1 00th
v ib ra tio n ; an d one th o u s a n d th o f th e tim e o f v ib ra tio n h a s be en
ta k e n th ro u g h o u t th e series as th e corre c tio n to in fin itely small a rcs.
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