The times. [volume] (Richmond, Va.) 1890-1903, 1891-02-11
Standard Tire.

No timepece Is perfect. and there ale nc
means on earth of keeping perfect time. The
stars however. furnish the necessary means.
At the observatory in Cambridge, there are
two principal clocks employed IN keeping the
standard time: the standard means time
clock, which telegraphs It's signals over the
surrounding country. and the normal sidereal
clock. which j. the main standard Of the OBJ
seryatory. 10 which everything is referred
This sidercal clock. as its name implies. keeps
sidereal or star time which gains about three
minutes and fitty-eight seconds per day over
mean solar time, with which We are all
familiar.
The clock is Of the finest workmanship. and
is kept in brick vault underneath the obser-
yatory, where the temperature Is as nearly con-
stant as possible.
Every effort is made to protect it from any
influence which might effect its rate or, IN
other words the amount Of its gain or loss per
day. This il necessary in order that the
rate may be depended upon 10 give the
correct time during spells Of cloudy weather,
when no observations can be made
On every clear evening the error of this
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 cannon and sup
ported by pier Of solid masonry. II Is so ar
ranged that it can be directed toward any
point on the meridian. but cannot be pointed
away from the meridian line. On looking
into the eyepiece OF this telescope one sees "A
series Of fine parallel lines running north and
south across the field of view. the middle line
marking the meridian.
When time observation is 10 be made the
observer first selects suitable star from u
printed star-list. 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
on the meridian tointercept the star. and put
ting his eye 10 the eyepece, waits for the star
10 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 in
strument called chronograph. on which are
also being recorded the seconds of the Slderral
clock. The chrOnograph consists Of u cylinder
made TO revOlve by clock work, at the rate of
about once a minute.
A paper wrapped around this cylinder. re
ceiyes the record traced by a pen. which j.
connected with an electro magnet IN such "a
manner that any signal made either by the
operator or the clock. causes the pen 20 make
mark on the paper.
By examining this paper the observer Is
able 20 tell. within tenth of 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 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 10 be fast a cores
pOndlng weight is removed. making the clock
lose slowly. I'm that way the standard signals
are kept within few tenths of a second OF
the correct time.-Youths' Companion.