An introduction to astronomy: designed as a text-book for the use of students in college. By Denison Olmsted ...

'If t'tI l'f MStOON.
of seic';.-'c, eonsidti'ilng tile vast amounllt of plroperty and of
llhumanl life involved in naviogatio)l.
273..'Vt/e (b'l//Tce 1 of Ionyqfo tefd lethzIeen, two f])/aes, mai y be
ffo;di by? (?;ii,melhodf btq WhicA w cW l MP, a 8scefOtith(AU di f'ence
0/ /At'i' /l:.:IC.al t oi:tb, a!9 t.hef stzfe istttt_, of ah?9ol'tte tf;i/e.
As the, cartl thinns oln its axis fromi weit to east) any place
tllat lies; eastwar(l of another will comle soonlr undllerl te stun,
r \will hlavo tt sin earllier o1 tle leridtlian, anllt consequicntly,
ini respect, to the 11hour- of the day, will be in adlvance of tile
othte att t le rate of olec ]ourti fot every 1.5~O or four minltes of
time finr' cl('|h de('ree. 1'ITus, to a pla(ce 1.i 5 east of Greetnw\lich,
it is It o'cloel(, P. Mr., whent it is 0noon. at (reenwi\ch; and to a
place I5~ west of that meridian, it is 1 o'clock, A. at. at the
samne instant. I enlee, the diffterenee of time at any two places
ildliecates t.lheir (iflerence of lonlitude.
2 41. if'thc easiest method of finding thie longitude is by
lleans of an a(ectrate t inepiclce, or c/tionometfr. Let us set
6d1t fr'1ou London, with a chronollol ter' accurately adjilstedt to
Greenwich -time, ii(ld travel eastw\ard to a certain pflace, \where
the titne is accurately kcpt, or may be ase lrtained by observa —
iontl. \et' find, f or exattnle,) that it is 1. o'clock by o01ur clhrolleter, whllen it is 2 o'clock and 30 minutes at tlle place of
oi)scrvation. -ellnce, thc lo1nlitli(le is:I X-1.5.:..... 22. t Ilad'\e travceled Awestwnardl uiltil lour cl'Ollnonetet a' 111 was ahour and
a halfl il t( vanlce of tile titme at tle place of observationl (thiat
is, st 1nu11(1 iiater ir thlt (l-t), our ltongitlud e \would lIave been
a2?7.? WN. Blut it would\ not )e Ilnecessary to rel)air to L]ondon in
or'der' to set our c-lhronlometer to (l rcellichl time.:ltis 11siglht
be donee at; anlt oblservatolry, o' ittY place wlose longitulde had
tcn tacmratet (ly t dtlerminc(. For example, thle time. at New
)(Yor1k is t111. l.L. 5 behltind tllhat of (IGrelnwich. If, therefore,
C e t () lt ro''llCometen'r so mucll before' tle true timet at N"',r
Yolk, it, will indticate tile time at Greenwichl.,tMoreover\, Ol
arrliving, at (liltkermtt p1taces, an\ywhere oil tlle catil, whose
lol,,'itntdc is accuratelyl 1knowni0 we may learn whectther our01
(:tl'otl(tlnCtorl' l(eeIS accurate titme 01' nlOt; andti if' not, the
amounll t of its error.'Chrotlnometers lavl b)eenl cltitructed( of
se/t1 al astoniti ng( dg'1oe of aiaccsin (acy. s to eviate but a f't(
Weulacy,~ ~ (viate