The times. [volume] (Richmond, Va.) 1890-1903, 1891-05-10
The Wsv Time ia Computed.

No timepiece is perfect and there ere no
means on earth of keeping correct time. Inc
stars. however furnish the necessary means.
At the observatory in Cambridge there are
two PRINCIPAL clocks employed IN keeping me
standard time-the standard mean time CLOCK,
which teecraphs its signals over the sur
rounding country and the normal sldereal
clock. which is the main standard Of the OBJ
servatory. 20 which everything Is referred.
The sidereal clock, as its name lmpll 70, keeps
sidereal or star time, which gains about thTeo
minutes and fye-eighths seconds per day
over mean solar time, with which we are all
familiar
The clock is of the finest workmanship, and
is kent in a brick vault underneath the obser-
yatory, where the temperature Is as nearly
constant as possible.
Every effort IS made to protect it from any
influence which might affect Its rate, or IN
other words. the amount OF It's gam or loss
per day. This is necessary in order that the
rate may be depended uDOn to give the cor
rect time during spells of cloudy weather,
when no observations can be made.
On every clear morning the error OF TINS
clock is carefully determined by observing
certain bright stars with an instrument known
as the meridian circle.
This instrument consists of a telescope
mounted on trunniOns like q cannon and
supported by pier of solid masonry. 1t Is
s0arranged that it can be directed toward
any point in the meridian, but can not be
pointed away from the meridian line. On
looking into the eye-piece otthls telescope one
see series Of fine parallel lines running
north and south across the field OF view, the
middle line marking the meridian.
When a time observation is TO be made the
observer first selects a suitable star from "a
printed star lisf, which gives the exact time at
which each Of the principal stars crosses the
meridian He then sets the telescope at the
proper point on the meridian 10 intercept
the star. and putting his eye TO the eye-pece
waits for the star to appear.
AS the star crosses each of the lines before
mentioned he presses an electric key which
he holds in his hand. The signals thus given
are recorded electrically on a registering III
strument called chronOgraph, on which are
also being recorded the seconds of the ST.
dereal clock The chronOgraph consists of a
cylinder made 10 revOlve by clOckwOrk at the
rate Of about once minute.
A paper, wrapped around this cylinder, re
ceives the record traced by n pen, which is
connecsed by an electric magnet in such "a
manner that any signal. made either by the
operator or by the clock, causes the pen 10
make q mark on the paper.
BY examining this paper the observer is
able TO tell within the tenth of n second the
time which the normal clock indicated when
the star crossed the meridian.
Comparing this with the time taken from
the star list shows the error of the clock A
comparison is then made between the sidereal
and mean time clocks, which, after allowing
for the difference between the mean and
sidereal time, shows the error of the mean
time clock.
If the clock Is slow slight weight is placed
on the top of the pendulum bob. which causes
the clock to gain slightly If, on the other
hand. the clock is found to be fast, CORES
ponding weight is removed, making the clock
lose slowly In this way the standard signals
are kept within few tenths of a second of
the correst time.-Youth's Companion.