Mineral Point tribune. (Mineral Point, Wis.) 1869-1938, 1873-08-28

oval, and the earth moves slowly, and, as
we see the SUN is behind his time. 1t is
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
SUMS 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 mag
| 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 find rACcalyac IN, q
! queer little chamber, in which the most
prominent object is very beautiful speci-
! men OF clock whose pendulum, instead
| OF osciiating backwards and forwards,
! swings round in circle, thus producing
! motion perfectly uniform and unbroken.
! This clock is revolving the H"chrOnO-
| graph" which consists of a cylinder
around which a 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 1t 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, il
plays an important part in registering OBJ
servations. Besides this it regulates sew.
eral clocks within the Observatory, as well
as the large one already referred to out
side the gates one al Greenwich Hospital
Schools, another at the London Bridg
Station Of the South. Eastern Railway
another at the Post Office, st. Martin sic
| Grand, and another in Lombard Street
Once every day it telegraphs correct
time to the great clock tower at West
minister; ii drops the signal ball over the
! Observatory, another near Charing Cross
and one at Deal; it fires time guns al
Shields and Newcastle, and every hOul
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
ont any human interference whatever
Every morning it is corrected by an actual
observation Of a star; and thus, without
being aware of il, 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 sUrmOunting 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, it is plain that 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 a 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
yeioped by theory, can be compared with
observation" Nensurative astronomy
for practical purposes is the great btsi
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
and of It's kind is one of the finest in the
world It's object glass, which is thirteen
inches in diameter, and has a focal dis
tance of eighteen feet, alone cost 81, 200
The most curious feature in this telescope
is the clockwork rangement by which
it follows any object under examination
Ii is used, as already intimated, chiefly
for what may be called gazing purposes