Public ledger. [volume] (Memphis, Tenn.) 1865-1893, 1873-08-20
GREENWICH TIME.
Il. WHITMORE,
Publisher and Proprietor

From Cassels s MaBaLlno.I
Never had science a more pleasant re
treat than Greenwich Observatory ap
pears to be this bright summer morning
For all its pleasant aspect, however
the idea OF exploring il is decidedly 9
formidable one. At the very entrance
gates one feels suddenly convicted ot
the most abject ignorance. Here are
mysterious metal pins fixed on the wal
for the determination of British meas
urements, and the question at once
arises, What have these to do with as
tronomy? Then there is great clock
dial, on which the hours are reckoned
frem one to twenty-four, and which is
popularity believed to be kept going by
the sun.
Determined to clear the way as he
goes on, the visitor makes these outer
difficulties the subjects Of his first in
quiries on gaining admittance, and he
discovers to his amazement that the
very length of his trousers and the cut
of his coat, and the hight of his hat,
have all been determined by measure
ments, based upon the motions of the
heavenly bodies.
A tailors yard measure, it appears,
bears certain proportion to the length
of a pendulum which, under specified
conditions, beats accurate seconds of
time, and seconds of time are deter
mined by astronomical observation. If
the taller wishes to verify his measure,
he has only to bring it to the Observa-
tory gate, where he will find a standard
absolutely accurate. As to the clock,
it is an astronomer clock, and astron-
omers know nothing Of u. m. and p. m. S;
their calculations are sufficiently com
plicated without them. The notion that
it is kept going by the sun is, it need
hardly be said delusion.
On passing the outer portal of the OBJ
servatory, the visitor finds himself in an
open courtyard, with an irregular pile
of buildings on his left hand. En-
tering a low doorway in one of these,
he is at once interested to discover that
he is really at what may be considered
the fountain.head of all our computa-
tions of time. The chief business of
Greenwich, as all the world knows, is to
tell US the time of day, and in this small
and somewhat mean looking apartment
is the great telescope by which observa-
tions for this purpose are effected.
This instrument-the transit circle, as
it is technically called-is twelve feet in
length, and its largest glass is eight
inches in diameter. 1t is suspended by
the middle between two massive stone
buttresses in such manner as to per-
init of its sweeping the sky in straight
line overhead, though it cannot be veered
round to the right or left.
We have arrived, let us suppose, lit
tie before noon the sun is about to
cross the meridian, and an observation
is to be made. Shutters in the roof are
thrown open, the great telescope is
swung up and fixed in position, and an
observer seats himself at the lower end
of it. While we are waiting for the
great luminary, let US take a peep
through the instrument. All that can
be seen is q number OF vertical lines
technically called wires, though they are
in reality so many pieces of cobwebs-
stretched across the field of observation
A. irregular distances. The center one
is the celebrated meridian OF Greenwich,
or at all events it represents it, and 1t is
curious to reflect that from this center
line ships of all civilized nations, and
in all parts of the known world, are
reckoning their distances; that this little
piece of cobweb is, practically, all that
divides the world into eastern and vest
ern hemispheres.
While we are peeriug along the tele
scope, the drogsy tinkling of innumer-
able clocks is heard through the still
summer AIR, and we begin 10 think that
for once at least the sun is behind time.
If not, then it seems plain that all the
Greenwich clocks are wrong, a supposi-
ton which is quite at variance with all
our fractional ideas of the place. On
INQUIRY. it is gratifying to find that our
faith in Greenwich time-pieces is per-
feetly justified, and that it really is the
sun that is behind time. The apparent
motion Of the suR. ss everybody knows,
is really the motion Of the earth. Now
the earth moves round the sun Ia s kind
Of oval pathway. When she is on either
s;da of this oval her motions are accele-
rated. and the sun will cross the merid-
ian before be is due. just now, how
ever. we are at one eui Of the eval, and
the earth moves slowly, and, as &c see,
the sun is behind his time. 1t IS clear,
therefore. that il the Greenwich clocks
vere 10 be regulated according to the
tiue At which The lord Of day puts in an
snnearauce at this Little cobweb, they
vould require constant alteration. They
srs however, set to record the average 4
me M his transit This never varies, /
sn5 twelve o'clock Grsenwith mean I
t;me" IL simply the mean or average
tine Al which throughout the year the
sun crosses the meridian
Let the observer nov resume his watch
Al The instrument What be has to DO