Images(3)

Mohn cloud theodolite

Datecirca 1885

Inventory Number1996-1-0111

ClassificationTheodolite

Subject

Maker Norwegian

Inventor Henrik Mohn 1835 - 1916

User Blue Hill Meteorological Observatory 1885-present

Cultural Region

Place of Use

Dimensions73.5 × 49 × 37.5 cm (28 15/16 × 19 5/16 × 14 3/4 in.)

Material

Bibliography

Blue Hill Meteorological Observatory: The First 100 Years---1885-1985

DescriptionCloud theodolite. A brass instrument consisting of a three-footed base with leveling screw feet. A pedestal supports a positionable sighting element. The sighting element has a conical shape and consists of a closed eyepiece at one end and an open cage at the other end. Horizontal and vertical axes enable adjustment in azimuth and altitude. The measurements are taken on two divided circles, each with a vernier, subdivided from 0 - 60 arc minutes. The eye piece contains cross hairs, but no optics, and the open end has cross wires for aligning the direction of sight.

The instrument has a glass bubble level.

Signedunsigned

Curatorial RemarksPossibly for use with plane table #1996-1-0113 (currently missing).

FunctionCloud theodolites were used in pairs in order to visually sight clouds and determine their positional angles or heights in relation to the observers. Successive observations may be performed to determine cloud velocity and direction. They were also used to study auroras.

ProvenanceFrom the Blue Hill Meteorological Observatory, Milton, MA

Historical AttributesThe cloud theodolite was invented by Henrik Mohn (1835-1916) of Norway. He was the teacher and colleague of a Norwegian scientist and meteorologist, Axel Severin Steen (1849-1915), who employed a similar cloud theodolite at the Norwegian Polar Station in Bossekop during the First International Polar Year (1882-1883). This cloud theodolite was one of a pair used by the Blue Hill Meteorological observatory to obtain the first routine measurements of cloud heights in North America in 1885-1886. It was built in Norway for $100. The data was used as a basis for subsequent climatology of cloud heights.