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Airy, Sir George Biddell

measurements gravitational observed model

[ ay ree] (1801–92) British geophysicist and astronomer: proposed model of isostasy to explain gravitational anomalies.

Airy was successful early in life, his talent and energy leading to his appointment as Astronomer Royal in 1835, a post he held for 46 years. He much extended and improved the astronomical measurements made in Britain. Airy’s researches were in the fields of both optics and geophysics. He experimented with cylindrical lenses to correct astigmatism (a condition he suffered from himself); and he studied the Airy discs in the diffraction pattern of a point source of light.

In geophysics he proposed that mountain ranges acted as blocks of differing thickness floating in hydrostatic equilibrium in a fluid mantle, rather like icebergs in the sea. He was thus able to explain gravitational anomalies that had been observed in the Himalayas as due to the partial counteraction of the gravitational attraction of the topography above sea level with that of a deep ‘root’ extending into the mantle. His model of isostasy satisfactorily explains the gravity field observed over mountainous terrain in much of the world.

Airy was arrogant and unlucky in his failings, now almost better known than his successes. He failed to exploit prediction of a new planet, Neptune; he was against idea of ‘lines of force’ (a fruitful intuition, in fact); and although he expended great effort to ensure precise measurements of the transits of Venus, observed in 1874 and 1882, the results failed to give accurate measurements of the scale of the solar system because Venus’s atmosphere makes the timing of its apparent contact with the Sun’s disc uncertain. An ingenious inventor of laboratory devices, he was remarkably precise, to the extent of labelling empty boxes ‘empty’.

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