© Museum of the History of Science, Oxford
Protractors are used for drawing and measuring lines at different angles. They were first invented as separate instruments rather than parts of other instruments in the early 17th century, when they were used on sea charts by sailors. Later, more specialised forms of protractor were invented, such as circular protractors used by astronomers. Modern protractors are usually semicircular in shape and have a scale on them that is divided into 180 degrees.
This protractor is especially designed for architects. As well as a scale of 180 degrees it also has a scale that goes from III at the 60 degree point to XII at the 105 degree point. This scale is used for drawing polygons. The III at 60 degrees is for drawing equilateral triangles, because they have corners with angles of 60 degrees. The XII is for drawing regular dodecagons (plane shapes with twelve equal sides), which have internal angles of 150 degrees. All the regular polygons in between the triangle and the dodecagon can also be drawn.
The protractor is made of solid silver and is top quality. It comes with its own case which matches the shape of the protractor exactly. The case is lined with silk and covered with 'shagreen'. Believe it or not, although usually made from leather, 'shagreen' in this case is actually fish skin which has been dried, dyed and polished. The pattern of small circular dots that can be seen on the surface of the case represent the scales of the fish. Shagreen was used a lot in the 18th century to decorate scientific instruments, especially microscopes.
The letters 'C R' are marked on the back of this protractor, along with a crown symbol. This shows that it was made specially for Queen Charlotte. Queen Charlotte was the wife of George III, who was King of England from 1760 to 1820. Why Queen Charlotte had an architect's protractor is a bit of a mystery, since she was not very interested in science. King George III, on the other hand, is famous for having been very interested in scientific things. He collected scientific instruments and had a very large collection which is now on display in the Science Museum in London. He was also interested in the important scientific issues of his time, such as the problem of finding longitude.
George III probably gave this protractor to his Queen as a present because he liked it. Whether she liked it as much as he did we shall probably never know.