174: REPORT— 1896. 
for any exposure from three seconds down to the one-sixtieth ; but. in 
practice it is found that the exposures required are always very brief, and 
a latitude of exposure from a quarter of a second downwards would be 
ample. This is with the diaphragm aperture about a quarter of an inch 
in diameter, and it is evident that variation in this would afford the 
equivalent of much greater variation in exposure. The shutters also 
suffer from the fact that the sliding portions are made of ebonite, which is 
liable to warp in consequence of the high temperatures sometimes pro- 
duced in the interior of the camera-box when exposed to a hot summer 
sun. Shutters like those at Kew, or with aluminium sliding parts, would 
probably be better. 
The electrical exposing connections proved to be a great source of 
trouble. The site is on hard Permian sandstones and breccias, which are 
very dry, and so hard that it would have been very costly to have made a 
large hole to be filled with coke. After several trials a satisfactory earth 
was obtained by leading the terminal to the end of about 50 yards of 
copper wire, such as is used by bell-hangers, and twisting this to and fro 
in a trench in the surface of the rock, which was then filled in with soil and . 
turfed over. 
The electro-magnets on the cameras, however, required a fairly strong 
current to make sure that they would act, so the primary current from 
the discharging key works two relays of similar construction, placed one | 
in each camera-box, which simultaneously close local independent circuits 
and release the shutters. 
Another source of trouble has been the batteries. Those used were 
of the dry-cell type. But during the past summer they were found to 
fail several times, the moisture essential to their working being apparently 
driven off by the excessive heat and drought to which they were exposed. 
If they could be placed in a more substantial structure, which could be 
kept cooler, they would doubtless do better. Your Committee propose to 
replace them by Leclanché cells next year. 
The plates used have been those already found to give excellent 
results for ordinary cloud photography, namely, Mawson and Swan’s photo- 
mechanical plates, or those prepared by the same firm for transparencies. 
They are carried in double dark slides of the ordinary pattern, two of 
which are provided for each camera ; but those belonging to the camera at 
one end of the base are slightly thicker, and differ in other ways from 
those used at the other end, so that there is no possibility of mistaking 
them after exposure, and they cannot be used for the wrong camera. 
On the right-hand side of the central part of each camera is a small 
view-finder, in which a minute image of the view is projected on ground 
glass, and which is adjusted once for all, so that the view in the finder 
corresponds with that on the plate. 
A loose piece of black velvet for each camera completes the apparatus. 
Two observers are required, one for each camera, and in making the 
observations the Secretary to your Committee has been assisted by Mrs. 
Clayden, or by his brother, Mr. C. E. Clayden. 
Each observer is provided with three small flags—pink, blue, and 
yellow (to avoid railway colours), by means of which a simple code of 
signals can be made. For simplicity, let us call the observers A and B, 
and suppose A directs the observations and B can close the key which 
will effect the exposure. A watches the sky until a favourable opportunity 
seems to be approaching. He then puts up the yellow flag and places a 
