Black bulb thermometer. Mercury, large black bulb, graduated 0-180 degrees Fahrenheit. With an index and magnet in wooden box.

Black bulb thermometers were developed by chemist Richard Watson (1737-1816) in 1772 to measure solar radiation. Surrounded by an evacuated glass sphere, the black bulb was placed in direct sunlight. A similar instrument with a plain mercury bulb (‘bright bulb thermometer’) was used for comparison. This type of instrument was superseded by more accurate devices such as the pyranometer and actinometer.

This particular instrument may have been used as part of the Royal Observatory’s expedition to Scandinavia to view the total solar eclipse on 28 July 1851. Astronomer Royal George Biddell Airy (1801-1892) set up an observing station at Gothenburg (‘Göttenburg’) while his assistant Edwin Dunkin (1821-1898) observed from Oslo (‘Christiania’) in Norway. Former Royal Observatory Assistant George Humphreys (1822-1893) and his friend John Miland (n.d.) set up another observing station at Kristianstad (‘Christianstadt’) on the southern Swedish coast. All three parties focused on observing prominences and light dispersion during totality.

In addition, Dunkin and Humphreys took series of temperature readings before, during and after the eclipse. They used a syringe to extract the air from the glass sphere surrounding the black bulb thermometer and positioned the instrument in direct sunlight to measure cloud density in relation to solar radiation. Dry and wet bulb thermometers were positioned in the shade as a comparison. Dunkin recorded a maximum difference of 29.5 degrees Fahrenheit between the two types of thermometers while Humphreys’ instruments only showed a maximum difference of 15 degrees Fahrenheit.

The labels ‘203’ and ‘Humphrey’s black bulb thermometer’ on the outer box suggest that this may have been the instrument used by Humphreys during this expedition, assuming that the box and instrument have been correctly linked.

For a full account of the eclipse as seen by various observers, see ‘Total Eclipse of the Sun on July 28, 1851’, Monthly Notices of the Royal Astronomical Society, 21:1-115.