The times. [volume] (Washington [D.C.]) 1897-1901, 1901-05-05

which power is constantly derived Along
| the wall are several large pendulum
| clocks with no dial, the works showing
, somewhat pathetically behind the glass
| fronts where the dials ought 10 be. On
| the rod of each pendulum is little shelf
| on which are placed small brass scale
| weights, an ingenious device of astrono-
mers to raise or lower the centre of gray
| ity al the pendulum-or in other words
| to shorten or lengthen it, making it beat
| faster or slower-without disturbing it or
the works, as might be the case If the
DoD were moved up and down by the
| screw nut below 1t. A clock is first ap-
| proximateiy regulated by the nut and
/ then exactly regulated by the weights to
SUIT whatever instrument ii is used to
control.
On the bottom of each pendulum rod is
fastened 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
l a circuit by direct contact. This device
IS nothing more or less than child
horseshoe or permanent magnet, with its
familiar red paint and all. Below il is
hXed armature of soft iron on the end OF
spring and. as the magnet goes back
and forth n passing, it attracts the arma-
ture which in rising is arrested in little
apparatus similar to a Morse telegraph
relay. thus making a circuit, which is
broken as the magnet moves on ana the
spring pulls down the armature. Each
of these clocks operates an instrument:
that is to say. regulates the clock-work
which drives a visual Photographic, or
photometric telescope as the case may
be, keeping ii fixed on a star. Each clock
is carefully regulated to work with a
given instrument and a table of weights
prepared so that by putting on or taking
off the weights on the pendulum shelf a
clock will keep given instrument exact
ly following a star.
Besides these clocks, there is also, an
instrument originally designed and lately
greatly improved by Mr.. Gerrish. 1t is
called a 'commutator. and is 4 clock
work device with horizontal shaft car
rying brass discs. which make 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
simultaneously 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 hour
during the night the shutter on the cam
era attached to the 15-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
the plate a trail of 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 time, completely around covering
two degrees at kick as 1t photographs
the stars in its meridian all night long.
Another disc keeps telescope pointed
for ten minutes on a star, then releases IL
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 a revolving dark
companion, are photographed.
Other discs move the plate instead of
the telescope, producing the same effect,
and each disc does its own work on time
and without failure. And ali the discs
travelling together are electrically driven
by n clock carefully regulated by compar-
ison with the standard. All the instru-
ments, from the five-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,4are applied to the same kind of
work the year round under the Southern
Cross in far-distant Peru, and just now
in halfway Jamaica.