262 PROCEEDINGS OF SECTION A. 
This is the system adopted in Melbourne. To obtain the 
absolute velocity of the cloud, we have only to take a second 
picture at one or both stations after a short interval of (say) one 
minute, and measure the displacement of the image which, 
together with the known height and focal length of the apparatus, 
determines the actual distance traversed by the cloud during the 
interval between the exposures. I will now proceed to describe 
the Melbourne arrangements for carrying out this work. I have 
here a pattern of the body of the cameras, which will give you an 
idea of their shape and dimensions. 
The instruments are made of heavy cast-iron, and are similar 
in every respect. Each consists, generally speaking (and disre- 
garding particular detail), of a hollow frustrum of a pyramid with 
a square base, and having a circular opening at the top, into 
which the lens is fitted. 
Once adjusted, the lens is permanently fixed, and intended to re- 
main in an invariable position with regard to the body of the camera. 
A shallow rectangular opening at the bottom of one side of the appa- 
ratus serves to introduce the dark slide which carries the plates. 
On the inner face of the base there are two guides, one of 
which is plain, and one A-shaped on the top. On these rests the 
dark slide when zz situ. I have here, also, one of the dark slides 
for your inspection. 
You will notice that the wooden parts are attached to a sub- 
stantial brass frame, one side of which has a v-shaped groove all 
along its length, with a stop at one end. This groove fits into 
the guide within the camera, and thus the slide always occupies 
an invariable position in respect to the body of the instrument. 
The plate, which is a square, whose side is 158 mm., has two of 
its contiguous edges grounded with a rounded corner between. 
When ready for exposure it rests on three steel points, which 
are fixed on the brass frame, and its two grounded edges are 
brought in contact with three cylindrical studs, also fixed on the 
brass frame, while two springs on the opposite sides keep it in 
that position. So that the plate occupies always an invariable 
position in respect to the slide, and consequently also in respect 
to the body of the camera. 
The fiducial lines, which are required to determine the centre 
of the plate and its orientation, are obtained by means of a latent 
image of a reseau impressed on the plate, which is developed 
together with the cloud picture. 
The reseau consists of two sets of parallel lines, 5 mm. apart, 
one set being at right-angles to the other. The intersection of 
the middle lines of the reseau form the centre of the plate. 
The reseau process is exactly the same as it is practised in the 
astrophotographie work of the Observatory, and it is therefore 
unnecessary to describe it. 
