The times., 1901-05-05

which power is constantly derived Along
/ the wall are several large pendulum
| clocks with no dial the works showing
| somewhat patheticaIly behind the glass
| fronts where the dials ought 20 be On
! the rod of each pendulum is a little shelf
! on which are placed small brass scale
! weights an ingenious device of astrono-
merS to raise or lower the centre of grav-
| tty al the pendulum-or. in other words
| to shorten or lengthen it, making it beat
raster or slower-without disturbing it or
| the works, as might be the case If the
! bob were moved up and down by the
screw nut below IL. A clock is first ap-
proXimately regulated by the nut and
then exactly regulated by the weights to
suit whatever instrument il is used to
control.
On the bottom of each pendulum rod is
fastened a little device which originated
at Cambridge. and which is designed to
! avoid the friction in the case of the Ordi-
nary pendulum. which makes and breaks
circuit by direct contact. This device
Is nothing more or less than a child
/ horseshoe or permanent magnet, with it's
familiar red paint and all. Below II IS
fixed armature of soft iron on the end of
spring and. as the magnet goes back
and forth = passing, it attracts the arma-
ture which in rising is arrested in a little
apparatus similar to a Morse telegraph
relay. thus making a circuit. which Is
broken as the magnet moves on and the
spring pulls down the armdture. Each
of these clocks operates an instrument;
that is TO say, regulates the clock work
which drives a visual, Photographic, or
photOmetrtc telescope, as the case may
be, keeping it fixed on a star. Each clock
is carefully regulated TO work with
given instrument and q table of weights
prepared, so that by putting on or taking
off the weights on the pendulum shelf a
clock will keep a given instrument exact
ly following star.
Besides these clocks, there is also, an
instrument originally designed and lately
greatly improved by Mr.. Gerrish. 1t is
called 'commutator," and is q clock
work device with a horizontal shaft car
rying brass discs, which make a complete
revolution in one hour, moving every min
ute, and regulated by one of the clocks.
Resting against the edge of each disc is a
little lever with miniature rubber wheel
to reduce friction. Where the disc is sol
id the lever does no work, but where it is
cut away the lever drops forward and es-
tabliShes an electrical connection, taking
the place of the finger in the key in TAX
ing sidereal time. Thus the ten discs 81
multaneously control ten different pieces
of apparatus, scattered all over the
grounds, and yet, with far greater accu-
racy than any person could pretend to Sc
cure
For instance. as now arranged. the first
disc has slots in it so that once an hour
during the night the shutter on the cam
era attached to the I5-inch reflector or
mirror, far out on the grounds, opens for
two minutes, closes for two minutes,
opens for one minute. closes for two min
uteS, opens for two minutes, and then
closes for the rest of the hour. leaving on
the plate a trail of a circumpolar star, for
example like this: -
The next disc has three slots which
every twenty minutes set off the funny
little transit photometer, the local clock
work of which sets it kicking two seconds
at d time, completely around, covering
two degrees at a kick as it photographs
the stars in its meridian all night long.
Another disc keeps a telescope pointed
for ten minutes on a star. then releases ii
for minute. in which period there is not
time for the star TO make an impression
on the plate, and then arrests the camera
for another ten minutes and so on all
night, making a series of photographs,
like successive dots, of a star 1t Is by
these plates that variable stars where
the light is cut off by revolving dark
companion. are photographed.
Other discs move the plate instead OF
the telescope, producing the same effect,
and each disc does it's own work on time
and without failure. And all the discs
travelling together are electrically driven
by clock carefully regulated by compar-
ison with the standard. All the instru-
ments, from the fiYe-inch transit photom-
eter to the eight-inch and eleven-inch
Draper Photographic telescopes, are pho
tographing the sky every night at Cam
bridge. and the same methods, planned at
Cambridge, with instruments built in this
country, are applied TO the same kind of
work the year round under the Southern
Cross in far-distant Peru, and just now
in halfway Jamaica.