Miner's dial, c 1850.
© Science Museum / Science & Society Picture Library
Magnetic compases are portable and easy to use for rapid survey were high accuracy is not required. In mining areas this adaptation of the compas to include a telescope above the dial was extremely popular for use underground. Henry Barrow, founder of Barrow and Co, the manufacturers of this instrument, had worked in India with Sir George Everest in the 1830s.