Standard mercury barometer, Newman No. 64, with vernier scale and two attached thermometers. Fixed to a wooden mount. This was the standard barometer at the Royal Observatory, Greenwich, from 1840 until 1950.

Inscribed "I. NEWMAN/ 22 Regent St:/ LONDON"

The thermometer component is signed by a different maker (Negretti and Zambra) and is most likely a later replacement. Negretti and Zambra took over Newman's business in 1862.

By the 1870s, astronomers had noticed a discrepancy between the barometer readings at Greenwich and elsewhere. In April 1877, a series of 3 experiments were carried out to compare barometers at Kew and Greenwich observatories and a small difference of -0.006 inches was indeed detected. Astronomers concluded that this standard barometer may have been affected by the removal of a long sliding rod on 20 August 1866 and so the correction was applied to all results from that date.

Similarly, the testers from Kew Observatory noticed a discrepancy in the readings of the thermometers positioned alongside the barometers at Greenwich. After some investigation, the observers realised that the gas burners used to illuminate the instrument scales were heating up the surrounding air. Protective glass screens were subsequently installed to avoid any further anomalies.

For more details see the following:
Cornes et al (2012), ‘A daily series of mean sea-level pressure for London, 1692-2007’, International Journal of Climatology, 32: 641–656.
Whipple, G.M. (1878), ‘On the Comparison of the Standard Barometers of the Royal Observatory, Greenwich, and the Kew Observatory’, Proceedings of the Royal Society of London, 27:76-81.