HISTORY OF THE PERTH OBSERVATORY. 
This work, which up to now has constituted our main 
contribution to science, is divided between the two instru- 
ments in the following manner. 
With the astrograph photographs covering the region 
assigned to us are being taken. Each plate contains an 
area of two degrees square, and successive plates overlap 
so that the edge of one corresponds to the central line ol 
the next. Sufficient exposure is given to secure every object 
in the sky down to the nth magnitude, fn the Milky 
Way there are sometimes as many as 2,000 stars on a single 
plate, whereas in other portions of the sky there are less 
than 100. These are all measured and the positions referred 
to two straight lines on each plate as axes of co-ordinates. 
These measures, of course, only give relative positions, sub- 
ject to various errors due to incorrect orientation and scale 
values. In order to obtain the absolute places in the sky 
we must know the positions of, say, twelve standard stars 
on each plate. These are determined by means of the 
Transit Circle. There are thus two main parallel lines of 
work going on. The rectangular co-ordinates of all stars 
in the assigned region down to the nth Magnitude are 
being measured from the photographs, and the positions 
of about ten thousand selected stars are being rigorously 
determined by means of the transit circle. 
An important part of the practical work of the Ob- 
servatory is the distribution of standard time. The error 
of the sidereal clock is, of course, determined with the 
transit circle, and thence the standard time of the State 
is computed. This is distributed to the public in the 
following ways : — 
(1) A time gun is fired daily in the Observatory 
grounds at 1 p.m. (12 noon on Saturday). 
(2) A time ball is dropped daily at Fremantle at 
1 p.m. and at such other hours as may be 
required for the benefit of shipping. 
(3) A time gun is fired daily at 1 p.m. at Fremantle. 
(4) Several time balls or semaphores are dropped 
at various hours in the city of Perth. 
(5) An exact signal is sent at every hour to the 
chief operating room in the Perth telegraph 
office, which controls vaiious clocks. Thence 
a signal is sent daily at noon to every telegraph 
office in the State. 
(6) A similar control is effected in the central 
railway office, whence time is distributed daily 
to every State railway station. 
