Reflecting Alidade

Reflecting Alidade

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Sir Howard Douglas (1776-1861), a well-known military officer and author, received a British patent (#3461) for a "reflecting circle or semicircle" in 1811. William Cary, who made the first examples of this instrument, noted that it combined a protractor with a reflecting system as used in a sextant. Cary also noted that it was "particularly useful in military survey, where the true situations of objects can at once be determined, and the sketch corrected at the same time that it is taken." Instruments of this sort were still available at the end of the 19th century.
The "Dollond London" inscription on this example refers to either George Dollond (fl. 1820-1852), or to George Dollond II (fl. 1852-1866). The horizontal arc extends 140 degrees, is graduated to 30 minutes, and is read by vernier to single minutes. The straight limb has a linear scale which is divided into tenths by diagonals. This scale is not in Imperial or metric units, and differs from that in Douglas' original instrument.
Ref: Mr. Cary, "Description of the Patent reflecting Semicircle, invented by Sir Howard Douglas, Bart," Philosophical Magazine 38 (1811): 186-187 and plate VI.
"Howard Douglas" in Dictionary of National Biography, vol. 5, pp. 1203-1206.
Currently not on view
Object Name
alidade (reflecting)
Dollond, George
Dollond II, George
place made
United Kingdom: England, London
overall: 6 3/4 in x 4 1/2 in; 17.145 cm x 11.43 cm
overall: 1 7/8 in x 7 1/2 in x 5 in; 4.7625 cm x 19.05 cm x 12.7 cm
ID Number
catalog number
accession number
See more items in
Medicine and Science: Physical Sciences
Surveying and Geodesy
Measuring & Mapping
Data Source
National Museum of American History
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