Historical information
After years of precursory surveying, debate and proposals the most ambitious civil engineering project of the day, the Overland Telegraph Line, began construction in September 1870. Superintendent of Telegraphs, Sir Charles Todd led the construction through “terra incognita,” guided by the precursory surveys of John McDowall Stuart and technologies such as his prismatic surveying compass. The unknown and hostile landscape claimed the lives of several men and scores of transport animals in the dogged pursuit of telegraphic connection to the rest of the world.
Completed in August 1872, the Line connected Australia to the world via telegraph wires running 3,200 kilometres from Port Augusta in South Australia, to Darwin, then connecting via submarine cable to Java and beyond. The “earth [had been] girdled with a magic chain” according to the then Governor of New South Wales, Sir Hercules Robinson.
How does it work?
For use in surveying, the sight vane and prism are turned up on their hinge and the instrument is held horizontally either in the palm of one's hand or on a tripod. Two small discs of red and green glass attached to the prism can be flipped down over the sight line to reduce glare. The objective is to bring the subject into the sightline created by the prism, aligning with the thread of the sight-vane until the subject is bisected evenly. Once aligned, the division on the card may be read through the prism. This reading provides the magnetic azimuth, used for calculating the bearings of distant landmarks.
Physical description
Circular instrument mounted in a brass case with glass window and brass lid. The compass card face four black compass points printed on mint green paper; on the underside the magnetic needle would be affixed, all held in place by a brass knob at the centre. The arched labels of "Sawtell" and "Adelaide" and the Prince of Wales feathers appear to have been affixed with adhesive which has since yellowed in the areas of application on the compass card. The compass face is printed with numbers, every 10 degrees from 10 - 360, printed in reverse indicating this compass would have once held a mirror at the sighting bracket. On one side of the brass case is a brass hinged sighting-prism, possibly of ebonite. The sighting-prism is mounted in a hinged brass bracket on one edge of the brass case. It has two flip-type filter glasses (red and green) and folds down into a retracted travelling position. A hinged brass bracket on the opposite edge would have held the sighting bracket - carrying the sighting vane and mirror - which is now missing or removed. Under the hinge is a lever, possibly related to the movement of the bracket. Underneath the brass case is an indented circle with screw threads, possibly for attachment to a tripod, and indistinguishable marks scratched into the surface.
Inscriptions & markings
Etched on to the centre of the lid, "Sawtell ADELAIDE / No 792." Affixed to the paper compass face, possibly from separate pieces of paper, "SAWTELL / ADELAIDE" with the Prince of Wales Feathers above "SAWTELL". Underneath on remains of white tape in red: "159."
