Rare pelorus by the important American instrument makers Riggs and Brothers. The firm was started by a clockmaker, William Rigg  (1795-1861). The company changed its name from W. H. C. Riggs and Sons to Riggs & Brothers in 1865. Stylistically the instrument is mid-nineteenth century so was probably made about the date of the name change. The firm made chronometers and other marine instruments and went out of business in 1931.
Pelorus are now uncommon marine instruments. Essentially they allow the extremely accurate plotting of coastal features, either from on-board a ship or from significant observation points on the coast.
This instrument sits in a lidded mahogany box, the counter-balanced instrument is suspended in a gymbal, mounted on a pair of bronze cradles. A finely engraved compass rose fitted to the body of the instrument, with two adjustable spaces, one with an adjustable brake. The sighting posts, one with a carefully engineered slot and the other with a gnomen-like sighting wire are hinged for storage and transport. There is a black glass backgound screen, hinged for initial sighting, an unusual sophistication for these instruments.
The original mahogany box fitted with brass carrying handles and a pair of brass hook catches.
Case.  265 mm x 265 mm. 10 1/2 inches square.
Instrument. 225 mm diameter. 8 3/4 inches.
Good condition.