Iowa voter. [volume] (Knoxville, Iowa) 1867-1874, 1873-09-04

clear, therefore, that if the Greenwich
clocks were to be regulated according to
the time at which the lord of day puts in
an appearance at this little cobweb, they
would require constant alteration. They
are, however, set to record the average
time of his transit. This never varies,
and twelve O'Clock "Greenwich mean
time" is simply the mean or average time
at which throughout the year the sun
crosses the meridian.
Let the observer now resume his watch
at the instrument. What he has to do is
to record the precise instant at which the
suns edge or "limb," as astronomers ex-
press it, passes that central wire" In
any single observation, however, he may
be little at fault, and for the sake of
greater accuracy, therefore, he will note
the instant at which it passes over all the
"wires, and then strike an average be-
tween them.
Slowly the sun creeps up to the first
line, and the observer lightly taps little
spring attached to the telescope. The
second "wire" is reached, and again the
spring is tapped, and so on throughout
the whole of the seven or nine webs en-
ployed in the observation.
The spring is connected with a tele-
graphic wire extending to 'chrono-
graph" in a distant part of the building;
and in order to understand the method of
recording the observation, we will now
follow the telegraphic signal, or, as imag-
ination is even swifter than the telegraph,
we will imagine that we have reached the
"chronograph" first and are there ready
to receive the signals.
Accordingly we find ourselves in Q
queer little chamber, in which the most
prominent object is a very beautiful speci-
men of clock whose pendulum, instead
of oscil!ating backwards and forwards,
swings round in circle, thus producing
a motion perfectly uniform and unbroken.
This clock is revolving the "chrono-
graph" which consists of cylinder
around which sheet of white paper has
been strained. While we are watching
this revolving barrel, we see the observers
signals come. A little steel point, which
is travelling over the surface of the paper,
is in electric communication with the
spring attached to the great telescope;
and every time the observer taps the
spring, this little travelling point pricks
into the paper, thus recording that the
sun has Just crossed "wire." This in
ITSELF however, would not be a record of
the time of transit if it were not that
another little steel point, which is in con-
nection with a galvanic clock in another
part of the building, has previously mark
ed the sheet of paper into spaces repre-
senting precise seconds of time. On the
completion of the observation the paper
may be removed from the cylinder and
affords permanent record of it.
Nothing, perhaps, throughout the OBJ
servatory at Greenwich, is calculated to
strike the visitor with greater astonish-
ment than that galvanic clock to which
reference has just been made. There is
nothing very remarkable in its appear
ance, but the work it accomplishes renders
it perhaps the most wonderful clock in
the world, and certainly the most impor-
tant one in England.
In the first place, as we have seen, it
plays an important part in registering OBJ
servatons. Besides this it regulates sew
eral clocks within the Observatory, as well
as the large one already referred to out
side the gates; one at Greenwich Hospital
| Schools, another at the London Bridge
| Station of the South Eastern Railway,
another at the Post Office, st. Martn's-le-
| Grand, and another in Lombard Street.
Once every day it telegraphs correct
| time to the great clock tower at West
| minister; it drops the signal ball over the
| Observatory, another near Charing Cross,
and one at Deal; it fires time guns at
Shields and Newcastle, and every hour
throughout the day it flashes out correct
time to each of the railway companies.
All this is accomplished as it were by
the mere volition of the clock, and with
out any human interference whatever.
Every morning it is corrected by an actual
observation of a star; and thus, without
being aware of it, do we every day start
our trains, and make our appointments,
and take our meals by the motions of the
heavenly bodies as observed and recorded
during the preceding night.
We now proceed to one of those curl
Gus little dOmes surrounding various
parts of the Observatory. Here we find
AN instrument devoted entirely to the
study of the moon. Observations of the
moon are of immense importance to us
as a nation of navigators, inasmuch as
she affords the means of determining
longitude at sea. Her motions however,
from various causes, are Of an extremely
complicated nature, and it is very neces-
sary that she shall be observed at all
times, and under all circumstances. But
with the transit circle, the instrument
first noticed, 1T IS plain tnat the moon
could be observed only when she is cross
ing the meridian, and not always then
Some five or six-and-twenty years ago,
therefore, Sir George Airy, the present
Astronomer Royal, designed the "Altazi.
muth," and since then the Importance Of
Greenwich as lunar observatory has
been just about doubled.
With this instrument and the transit
circle the Observatory might do all that
strictly speaking, comes within its prov
ince. The whole duty Of Greenwich, as
defined by Herschel, is ''to furnish now,
and in all future time, the best and most
perfect data by which the laws OF the
lunar and planetary movements, as de-
veloped by theory, can be compared with
observation" Mensurative astronomy
for practical purposes is the great bust
ness of Greenwich.
The Great Equatorial telescope was
mounted about sixteen years ago, under
the direction and from the plans Of the
present Astronomer Royal. 1t is the
largest instrument in the Observatory,
opd of Ito kind il one of the finest in the