Photothermograph used at Radcliffe Observatory, Oxford

The photothermograph was an instrument designed for meteorological observatories that enabled a continuous photographic record of external wet and dry bulb temperatures to be taken, providing readings of atmospheric temperature and humidity.

The thermometers had a modified design so that they had an additional mercury column above the main mercury stem, separated by a small air gap; the long bent stems meant this air gap was indoors. Light from the gas lamps was focussed on the thermometer stems in the region of the air gap by a system of lenses and mirrors. The light passing through the air gap was focussed on the photographically sensitive paper of the recording drum, with the wet bulb recording below the dry bulb. A shutter blocked the light for four minutes every two hours, leaving time marks on the record.

The thermometers themselves were installed in an external louvred enclosure, similar to a Stevenson screen, which protected them from direct sunlight, wind and rain. Standard wet and dry bulb thermometers were also installed in the enclosure, providing readings that could be compared with that of the photographic record.

The photothermograph was an instrument designed for meteorological observatories that enabled a continuous photographic record of external wet and dry bulb temperatures to be taken, providing readings of atmospheric temperature and humidity.

The thermometers had a modified design so that they had an additional mercury column above the main mercury stem, separated by a small air gap; the long bent stems meant this air gap was indoors. Light from the gas lamps was focussed on the thermometer stems in the region of the air gap by a system of lenses and mirrors. The light passing through the air gap was focussed on the photographically sensitive paper of the recording drum, with the wet bulb recording below the dry bulb. A shutter blocked the light for four minutes every two hours, leaving time marks on the record.

The thermometers themselves were installed in an external louvred enclosure, similar to a Stevenson screen, which protected them from direct sunlight, wind and rain. Standard wet and dry bulb thermometers were also installed in the enclosure, providing readings that could be compared with that of the photographic record.

The photothermograph was an instrument designed for meteorological observatories that enabled a continuous photographic record of external wet and dry bulb temperatures to be taken, providing readings of atmospheric temperature and humidity.

The thermometers had a modified design so that they had an additional mercury column above the main mercury stem, separated by a small air gap; the long bent stems meant this air gap was indoors. Light from the gas lamps was focussed on the thermometer stems in the region of the air gap by a system of lenses and mirrors. The light passing through the air gap was focussed on the photographically sensitive paper of the recording drum, with the wet bulb recording below the dry bulb. A shutter blocked the light for four minutes every two hours, leaving time marks on the record.

The thermometers themselves were installed in an external louvred enclosure, similar to a Stevenson screen, which protected them from direct sunlight, wind and rain. Standard wet and dry bulb thermometers were also installed in the enclosure, providing readings that could be compared with that of the photographic record.

The photothermograph was an instrument designed for meteorological observatories that enabled a continuous photographic record of external wet and dry bulb temperatures to be taken, providing readings of atmospheric temperature and humidity.

The thermometers had a modified design so that they had an additional mercury column above the main mercury stem, separated by a small air gap; the long bent stems meant this air gap was indoors. Light from the gas lamps was focussed on the thermometer stems in the region of the air gap by a system of lenses and mirrors. The light passing through the air gap was focussed on the photographically sensitive paper of the recording drum, with the wet bulb recording below the dry bulb. A shutter blocked the light for four minutes every two hours, leaving time marks on the record.

The thermometers themselves were installed in an external louvred enclosure, similar to a Stevenson screen, which protected them from direct sunlight, wind and rain. Standard wet and dry bulb thermometers were also installed in the enclosure, providing readings that could be compared with that of the photographic record.

The photothermograph was an instrument designed for meteorological observatories that enabled a continuous photographic record of external wet and dry bulb temperatures to be taken, providing readings of atmospheric temperature and humidity.

The thermometers had a modified design so that they had an additional mercury column above the main mercury stem, separated by a small air gap; the long bent stems meant this air gap was indoors. Light from the gas lamps was focussed on the thermometer stems in the region of the air gap by a system of lenses and mirrors. The light passing through the air gap was focussed on the photographically sensitive paper of the recording drum, with the wet bulb recording below the dry bulb. A shutter blocked the light for four minutes every two hours, leaving time marks on the record.

The thermometers themselves were installed in an external louvred enclosure, similar to a Stevenson screen, which protected them from direct sunlight, wind and rain. Standard wet and dry bulb thermometers were also installed in the enclosure, providing readings that could be compared with that of the photographic record.

The photothermograph was an instrument designed for meteorological observatories that enabled a continuous photographic record of external wet and dry bulb temperatures to be taken, providing readings of atmospheric temperature and humidity.

The thermometers had a modified design so that they had an additional mercury column above the main mercury stem, separated by a small air gap; the long bent stems meant this air gap was indoors. Light from the gas lamps was focussed on the thermometer stems in the region of the air gap by a system of lenses and mirrors. The light passing through the air gap was focussed on the photographically sensitive paper of the recording drum, with the wet bulb recording below the dry bulb. A shutter blocked the light for four minutes every two hours, leaving time marks on the record.

The thermometers themselves were installed in an external louvred enclosure, similar to a Stevenson screen, which protected them from direct sunlight, wind and rain. Standard wet and dry bulb thermometers were also installed in the enclosure, providing readings that could be compared with that of the photographic record.

The photothermograph was an instrument designed for meteorological observatories that enabled a continuous photographic record of external wet and dry bulb temperatures to be taken, providing readings of atmospheric temperature and humidity.

The thermometers had a modified design so that they had an additional mercury column above the main mercury stem, separated by a small air gap; the long bent stems meant this air gap was indoors. Light from the gas lamps was focussed on the thermometer stems in the region of the air gap by a system of lenses and mirrors. The light passing through the air gap was focussed on the photographically sensitive paper of the recording drum, with the wet bulb recording below the dry bulb. A shutter blocked the light for four minutes every two hours, leaving time marks on the record.

The thermometers themselves were installed in an external louvred enclosure, similar to a Stevenson screen, which protected them from direct sunlight, wind and rain. Standard wet and dry bulb thermometers were also installed in the enclosure, providing readings that could be compared with that of the photographic record.

The photothermograph was an instrument designed for meteorological observatories that enabled a continuous photographic record of external wet and dry bulb temperatures to be taken, providing readings of atmospheric temperature and humidity.

The thermometers had a modified design so that they had an additional mercury column above the main mercury stem, separated by a small air gap; the long bent stems meant this air gap was indoors. Light from the gas lamps was focussed on the thermometer stems in the region of the air gap by a system of lenses and mirrors. The light passing through the air gap was focussed on the photographically sensitive paper of the recording drum, with the wet bulb recording below the dry bulb. A shutter blocked the light for four minutes every two hours, leaving time marks on the record.

The thermometers themselves were installed in an external louvred enclosure, similar to a Stevenson screen, which protected them from direct sunlight, wind and rain. Standard wet and dry bulb thermometers were also installed in the enclosure, providing readings that could be compared with that of the photographic record.

The photothermograph was an instrument designed for meteorological observatories that enabled a continuous photographic record of external wet and dry bulb temperatures to be taken, providing readings of atmospheric temperature and humidity.

The thermometers had a modified design so that they had an additional mercury column above the main mercury stem, separated by a small air gap; the long bent stems meant this air gap was indoors. Light from the gas lamps was focussed on the thermometer stems in the region of the air gap by a system of lenses and mirrors. The light passing through the air gap was focussed on the photographically sensitive paper of the recording drum, with the wet bulb recording below the dry bulb. A shutter blocked the light for four minutes every two hours, leaving time marks on the record.

The thermometers themselves were installed in an external louvred enclosure, similar to a Stevenson screen, which protected them from direct sunlight, wind and rain. Standard wet and dry bulb thermometers were also installed in the enclosure, providing readings that could be compared with that of the photographic record.

The photothermograph was an instrument designed for meteorological observatories that enabled a continuous photographic record of external wet and dry bulb temperatures to be taken, providing readings of atmospheric temperature and humidity.

The thermometers had a modified design so that they had an additional mercury column above the main mercury stem, separated by a small air gap; the long bent stems meant this air gap was indoors. Light from the gas lamps was focussed on the thermometer stems in the region of the air gap by a system of lenses and mirrors. The light passing through the air gap was focussed on the photographically sensitive paper of the recording drum, with the wet bulb recording below the dry bulb. A shutter blocked the light for four minutes every two hours, leaving time marks on the record.

The thermometers themselves were installed in an external louvred enclosure, similar to a Stevenson screen, which protected them from direct sunlight, wind and rain. Standard wet and dry bulb thermometers were also installed in the enclosure, providing readings that could be compared with that of the photographic record.

The photothermograph was an instrument designed for meteorological observatories that enabled a continuous photographic record of external wet and dry bulb temperatures to be taken, providing readings of atmospheric temperature and humidity.

The thermometers had a modified design so that they had an additional mercury column above the main mercury stem, separated by a small air gap; the long bent stems meant this air gap was indoors. Light from the gas lamps was focussed on the thermometer stems in the region of the air gap by a system of lenses and mirrors. The light passing through the air gap was focussed on the photographically sensitive paper of the recording drum, with the wet bulb recording below the dry bulb. A shutter blocked the light for four minutes every two hours, leaving time marks on the record.

The thermometers themselves were installed in an external louvred enclosure, similar to a Stevenson screen, which protected them from direct sunlight, wind and rain. Standard wet and dry bulb thermometers were also installed in the enclosure, providing readings that could be compared with that of the photographic record.

The photothermograph was an instrument designed for meteorological observatories that enabled a continuous photographic record of external wet and dry bulb temperatures to be taken, providing readings of atmospheric temperature and humidity.

The thermometers had a modified design so that they had an additional mercury column above the main mercury stem, separated by a small air gap; the long bent stems meant this air gap was indoors. Light from the gas lamps was focussed on the thermometer stems in the region of the air gap by a system of lenses and mirrors. The light passing through the air gap was focussed on the photographically sensitive paper of the recording drum, with the wet bulb recording below the dry bulb. A shutter blocked the light for four minutes every two hours, leaving time marks on the record.

The thermometers themselves were installed in an external louvred enclosure, similar to a Stevenson screen, which protected them from direct sunlight, wind and rain. Standard wet and dry bulb thermometers were also installed in the enclosure, providing readings that could be compared with that of the photographic record.

The photothermograph was an instrument designed for meteorological observatories that enabled a continuous photographic record of external wet and dry bulb temperatures to be taken, providing readings of atmospheric temperature and humidity.

The thermometers had a modified design so that they had an additional mercury column above the main mercury stem, separated by a small air gap; the long bent stems meant this air gap was indoors. Light from the gas lamps was focussed on the thermometer stems in the region of the air gap by a system of lenses and mirrors. The light passing through the air gap was focussed on the photographically sensitive paper of the recording drum, with the wet bulb recording below the dry bulb. A shutter blocked the light for four minutes every two hours, leaving time marks on the record.

The thermometers themselves were installed in an external louvred enclosure, similar to a Stevenson screen, which protected them from direct sunlight, wind and rain. Standard wet and dry bulb thermometers were also installed in the enclosure, providing readings that could be compared with that of the photographic record.

The photothermograph was an instrument designed for meteorological observatories that enabled a continuous photographic record of external wet and dry bulb temperatures to be taken, providing readings of atmospheric temperature and humidity.

The thermometers had a modified design so that they had an additional mercury column above the main mercury stem, separated by a small air gap; the long bent stems meant this air gap was indoors. Light from the gas lamps was focussed on the thermometer stems in the region of the air gap by a system of lenses and mirrors. The light passing through the air gap was focussed on the photographically sensitive paper of the recording drum, with the wet bulb recording below the dry bulb. A shutter blocked the light for four minutes every two hours, leaving time marks on the record.

The thermometers themselves were installed in an external louvred enclosure, similar to a Stevenson screen, which protected them from direct sunlight, wind and rain. Standard wet and dry bulb thermometers were also installed in the enclosure, providing readings that could be compared with that of the photographic record.

The photothermograph was an instrument designed for meteorological observatories that enabled a continuous photographic record of external wet and dry bulb temperatures to be taken, providing readings of atmospheric temperature and humidity.

The thermometers had a modified design so that they had an additional mercury column above the main mercury stem, separated by a small air gap; the long bent stems meant this air gap was indoors. Light from the gas lamps was focussed on the thermometer stems in the region of the air gap by a system of lenses and mirrors. The light passing through the air gap was focussed on the photographically sensitive paper of the recording drum, with the wet bulb recording below the dry bulb. A shutter blocked the light for four minutes every two hours, leaving time marks on the record.

The thermometers themselves were installed in an external louvred enclosure, similar to a Stevenson screen, which protected them from direct sunlight, wind and rain. Standard wet and dry bulb thermometers were also installed in the enclosure, providing readings that could be compared with that of the photographic record.

The photothermograph was an instrument designed for meteorological observatories that enabled a continuous photographic record of external wet and dry bulb temperatures to be taken, providing readings of atmospheric temperature and humidity.

The thermometers had a modified design so that they had an additional mercury column above the main mercury stem, separated by a small air gap; the long bent stems meant this air gap was indoors. Light from the gas lamps was focussed on the thermometer stems in the region of the air gap by a system of lenses and mirrors. The light passing through the air gap was focussed on the photographically sensitive paper of the recording drum, with the wet bulb recording below the dry bulb. A shutter blocked the light for four minutes every two hours, leaving time marks on the record.

The thermometers themselves were installed in an external louvred enclosure, similar to a Stevenson screen, which protected them from direct sunlight, wind and rain. Standard wet and dry bulb thermometers were also installed in the enclosure, providing readings that could be compared with that of the photographic record.

The photothermograph was an instrument designed for meteorological observatories that enabled a continuous photographic record of external wet and dry bulb temperatures to be taken, providing readings of atmospheric temperature and humidity.

The thermometers had a modified design so that they had an additional mercury column above the main mercury stem, separated by a small air gap; the long bent stems meant this air gap was indoors. Light from the gas lamps was focussed on the thermometer stems in the region of the air gap by a system of lenses and mirrors. The light passing through the air gap was focussed on the photographically sensitive paper of the recording drum, with the wet bulb recording below the dry bulb. A shutter blocked the light for four minutes every two hours, leaving time marks on the record.

The thermometers themselves were installed in an external louvred enclosure, similar to a Stevenson screen, which protected them from direct sunlight, wind and rain. Standard wet and dry bulb thermometers were also installed in the enclosure, providing readings that could be compared with that of the photographic record.

The photothermograph was an instrument designed for meteorological observatories that enabled a continuous photographic record of external wet and dry bulb temperatures to be taken, providing readings of atmospheric temperature and humidity.

The thermometers had a modified design so that they had an additional mercury column above the main mercury stem, separated by a small air gap; the long bent stems meant this air gap was indoors. Light from the gas lamps was focussed on the thermometer stems in the region of the air gap by a system of lenses and mirrors. The light passing through the air gap was focussed on the photographically sensitive paper of the recording drum, with the wet bulb recording below the dry bulb. A shutter blocked the light for four minutes every two hours, leaving time marks on the record.

The thermometers themselves were installed in an external louvred enclosure, similar to a Stevenson screen, which protected them from direct sunlight, wind and rain. Standard wet and dry bulb thermometers were also installed in the enclosure, providing readings that could be compared with that of the photographic record.

The photothermograph was an instrument designed for meteorological observatories that enabled a continuous photographic record of external wet and dry bulb temperatures to be taken, providing readings of atmospheric temperature and humidity.

The thermometers had a modified design so that they had an additional mercury column above the main mercury stem, separated by a small air gap; the long bent stems meant this air gap was indoors. Light from the gas lamps was focussed on the thermometer stems in the region of the air gap by a system of lenses and mirrors. The light passing through the air gap was focussed on the photographically sensitive paper of the recording drum, with the wet bulb recording below the dry bulb. A shutter blocked the light for four minutes every two hours, leaving time marks on the record.

The thermometers themselves were installed in an external louvred enclosure, similar to a Stevenson screen, which protected them from direct sunlight, wind and rain. Standard wet and dry bulb thermometers were also installed in the enclosure, providing readings that could be compared with that of the photographic record.

The photothermograph was an instrument designed for meteorological observatories that enabled a continuous photographic record of external wet and dry bulb temperatures to be taken, providing readings of atmospheric temperature and humidity.

The thermometers had a modified design so that they had an additional mercury column above the main mercury stem, separated by a small air gap; the long bent stems meant this air gap was indoors. Light from the gas lamps was focussed on the thermometer stems in the region of the air gap by a system of lenses and mirrors. The light passing through the air gap was focussed on the photographically sensitive paper of the recording drum, with the wet bulb recording below the dry bulb. A shutter blocked the light for four minutes every two hours, leaving time marks on the record.

The thermometers themselves were installed in an external louvred enclosure, similar to a Stevenson screen, which protected them from direct sunlight, wind and rain. Standard wet and dry bulb thermometers were also installed in the enclosure, providing readings that could be compared with that of the photographic record.

The photothermograph was an instrument designed for meteorological observatories that enabled a continuous photographic record of external wet and dry bulb temperatures to be taken, providing readings of atmospheric temperature and humidity.

The thermometers had a modified design so that they had an additional mercury column above the main mercury stem, separated by a small air gap; the long bent stems meant this air gap was indoors. Light from the gas lamps was focussed on the thermometer stems in the region of the air gap by a system of lenses and mirrors. The light passing through the air gap was focussed on the photographically sensitive paper of the recording drum, with the wet bulb recording below the dry bulb. A shutter blocked the light for four minutes every two hours, leaving time marks on the record.

The thermometers themselves were installed in an external louvred enclosure, similar to a Stevenson screen, which protected them from direct sunlight, wind and rain. Standard wet and dry bulb thermometers were also installed in the enclosure, providing readings that could be compared with that of the photographic record.

The photothermograph was an instrument designed for meteorological observatories that enabled a continuous photographic record of external wet and dry bulb temperatures to be taken, providing readings of atmospheric temperature and humidity.

The thermometers had a modified design so that they had an additional mercury column above the main mercury stem, separated by a small air gap; the long bent stems meant this air gap was indoors. Light from the gas lamps was focussed on the thermometer stems in the region of the air gap by a system of lenses and mirrors. The light passing through the air gap was focussed on the photographically sensitive paper of the recording drum, with the wet bulb recording below the dry bulb. A shutter blocked the light for four minutes every two hours, leaving time marks on the record.

The thermometers themselves were installed in an external louvred enclosure, similar to a Stevenson screen, which protected them from direct sunlight, wind and rain. Standard wet and dry bulb thermometers were also installed in the enclosure, providing readings that could be compared with that of the photographic record.

The photothermograph was an instrument designed for meteorological observatories that enabled a continuous photographic record of external wet and dry bulb temperatures to be taken, providing readings of atmospheric temperature and humidity.

The thermometers had a modified design so that they had an additional mercury column above the main mercury stem, separated by a small air gap; the long bent stems meant this air gap was indoors. Light from the gas lamps was focussed on the thermometer stems in the region of the air gap by a system of lenses and mirrors. The light passing through the air gap was focussed on the photographically sensitive paper of the recording drum, with the wet bulb recording below the dry bulb. A shutter blocked the light for four minutes every two hours, leaving time marks on the record.

The thermometers themselves were installed in an external louvred enclosure, similar to a Stevenson screen, which protected them from direct sunlight, wind and rain. Standard wet and dry bulb thermometers were also installed in the enclosure, providing readings that could be compared with that of the photographic record.

The photothermograph was an instrument designed for meteorological observatories that enabled a continuous photographic record of external wet and dry bulb temperatures to be taken, providing readings of atmospheric temperature and humidity.

The thermometers had a modified design so that they had an additional mercury column above the main mercury stem, separated by a small air gap; the long bent stems meant this air gap was indoors. Light from the gas lamps was focussed on the thermometer stems in the region of the air gap by a system of lenses and mirrors. The light passing through the air gap was focussed on the photographically sensitive paper of the recording drum, with the wet bulb recording below the dry bulb. A shutter blocked the light for four minutes every two hours, leaving time marks on the record.

The thermometers themselves were installed in an external louvred enclosure, similar to a Stevenson screen, which protected them from direct sunlight, wind and rain. Standard wet and dry bulb thermometers were also installed in the enclosure, providing readings that could be compared with that of the photographic record.

The photothermograph was an instrument designed for meteorological observatories that enabled a continuous photographic record of external wet and dry bulb temperatures to be taken, providing readings of atmospheric temperature and humidity.

The thermometers had a modified design so that they had an additional mercury column above the main mercury stem, separated by a small air gap; the long bent stems meant this air gap was indoors. Light from the gas lamps was focussed on the thermometer stems in the region of the air gap by a system of lenses and mirrors. The light passing through the air gap was focussed on the photographically sensitive paper of the recording drum, with the wet bulb recording below the dry bulb. A shutter blocked the light for four minutes every two hours, leaving time marks on the record.

The thermometers themselves were installed in an external louvred enclosure, similar to a Stevenson screen, which protected them from direct sunlight, wind and rain. Standard wet and dry bulb thermometers were also installed in the enclosure, providing readings that could be compared with that of the photographic record.

The photothermograph was an instrument designed for meteorological observatories that enabled a continuous photographic record of external wet and dry bulb temperatures to be taken, providing readings of atmospheric temperature and humidity.

The thermometers had a modified design so that they had an additional mercury column above the main mercury stem, separated by a small air gap; the long bent stems meant this air gap was indoors. Light from the gas lamps was focussed on the thermometer stems in the region of the air gap by a system of lenses and mirrors. The light passing through the air gap was focussed on the photographically sensitive paper of the recording drum, with the wet bulb recording below the dry bulb. A shutter blocked the light for four minutes every two hours, leaving time marks on the record.

The thermometers themselves were installed in an external louvred enclosure, similar to a Stevenson screen, which protected them from direct sunlight, wind and rain. Standard wet and dry bulb thermometers were also installed in the enclosure, providing readings that could be compared with that of the photographic record.

The photothermograph was an instrument designed for meteorological observatories that enabled a continuous photographic record of external wet and dry bulb temperatures to be taken, providing readings of atmospheric temperature and humidity.

The thermometers had a modified design so that they had an additional mercury column above the main mercury stem, separated by a small air gap; the long bent stems meant this air gap was indoors. Light from the gas lamps was focussed on the thermometer stems in the region of the air gap by a system of lenses and mirrors. The light passing through the air gap was focussed on the photographically sensitive paper of the recording drum, with the wet bulb recording below the dry bulb. A shutter blocked the light for four minutes every two hours, leaving time marks on the record.

The thermometers themselves were installed in an external louvred enclosure, similar to a Stevenson screen, which protected them from direct sunlight, wind and rain. Standard wet and dry bulb thermometers were also installed in the enclosure, providing readings that could be compared with that of the photographic record.

The photothermograph was an instrument designed for meteorological observatories that enabled a continuous photographic record of external wet and dry bulb temperatures to be taken, providing readings of atmospheric temperature and humidity.

The thermometers had a modified design so that they had an additional mercury column above the main mercury stem, separated by a small air gap; the long bent stems meant this air gap was indoors. Light from the gas lamps was focussed on the thermometer stems in the region of the air gap by a system of lenses and mirrors. The light passing through the air gap was focussed on the photographically sensitive paper of the recording drum, with the wet bulb recording below the dry bulb. A shutter blocked the light for four minutes every two hours, leaving time marks on the record.

The thermometers themselves were installed in an external louvred enclosure, similar to a Stevenson screen, which protected them from direct sunlight, wind and rain. Standard wet and dry bulb thermometers were also installed in the enclosure, providing readings that could be compared with that of the photographic record.

The photothermograph was an instrument designed for meteorological observatories that enabled a continuous photographic record of external wet and dry bulb temperatures to be taken, providing readings of atmospheric temperature and humidity.

The thermometers had a modified design so that they had an additional mercury column above the main mercury stem, separated by a small air gap; the long bent stems meant this air gap was indoors. Light from the gas lamps was focussed on the thermometer stems in the region of the air gap by a system of lenses and mirrors. The light passing through the air gap was focussed on the photographically sensitive paper of the recording drum, with the wet bulb recording below the dry bulb. A shutter blocked the light for four minutes every two hours, leaving time marks on the record.

The thermometers themselves were installed in an external louvred enclosure, similar to a Stevenson screen, which protected them from direct sunlight, wind and rain. Standard wet and dry bulb thermometers were also installed in the enclosure, providing readings that could be compared with that of the photographic record.

The photothermograph was an instrument designed for meteorological observatories that enabled a continuous photographic record of external wet and dry bulb temperatures to be taken, providing readings of atmospheric temperature and humidity.

The thermometers had a modified design so that they had an additional mercury column above the main mercury stem, separated by a small air gap; the long bent stems meant this air gap was indoors. Light from the gas lamps was focussed on the thermometer stems in the region of the air gap by a system of lenses and mirrors. The light passing through the air gap was focussed on the photographically sensitive paper of the recording drum, with the wet bulb recording below the dry bulb. A shutter blocked the light for four minutes every two hours, leaving time marks on the record.

The thermometers themselves were installed in an external louvred enclosure, similar to a Stevenson screen, which protected them from direct sunlight, wind and rain. Standard wet and dry bulb thermometers were also installed in the enclosure, providing readings that could be compared with that of the photographic record.

The photothermograph was an instrument designed for meteorological observatories that enabled a continuous photographic record of external wet and dry bulb temperatures to be taken, providing readings of atmospheric temperature and humidity.

The thermometers had a modified design so that they had an additional mercury column above the main mercury stem, separated by a small air gap; the long bent stems meant this air gap was indoors. Light from the gas lamps was focussed on the thermometer stems in the region of the air gap by a system of lenses and mirrors. The light passing through the air gap was focussed on the photographically sensitive paper of the recording drum, with the wet bulb recording below the dry bulb. A shutter blocked the light for four minutes every two hours, leaving time marks on the record.

The thermometers themselves were installed in an external louvred enclosure, similar to a Stevenson screen, which protected them from direct sunlight, wind and rain. Standard wet and dry bulb thermometers were also installed in the enclosure, providing readings that could be compared with that of the photographic record.