Reflecting Alidade
name: label: maker
content: Dollond, George
label: maker
content: Dollond II, George
notes: label: Description
content: 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.
label: Description
content: 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.
label: Description
content: Ref: Mr. Cary, "Description of the Patent reflecting Semicircle, invented by Sir Howard Douglas, Bart," <i>Philosophical Magazine</i> 38 (1811): 186-187 and plate VI.
label: Description
content: "Howard Douglas" in <i>Dictionary of National Biography</i>, vol. 5, pp. 1203-1206.
label: Location
content: Currently not on view
place: label: place made
content: United Kingdom: England, London
setName: label: See more items in
content: Medicine and Science: Physical Sciences
label: See more items in
content: Surveying and Geodesy
label: See more items in
content: Measuring & Mapping
publisher: label: Related Publication
content: Philosophical Magazine
label: Related Publication
content: Dictionary of American Biography
dataSource: label: Data Source
content: National Museum of American History
identifier: label: ID Number
content: PH.319452
label: catalog number
content: 319452
label: accession number
content: 238494
objectType: label: Object Name
content: alidade (reflecting)
physicalDescription: label: Measurements
content: overall: 6 3/4 in x 4 1/2 in; 17.145 cm x 11.43 cm
label: Measurements
content: overall: 1 7/8 in x 7 1/2 in x 5 in; 4.7625 cm x 19.05 cm x 12.7 cm