Five machine screws: three raised head, a countersunk and a cheese-head, for double grindstone model
Five machine screws from model of double grindstones, scale 1:4, by William Muir & Co., Manchester, 1853
Muir patented his double grindstone in 1853. The Dictionary of National Biography records: 'A model of his grindstone, patented in 1853 (No. 621), may be seen at South Kensington Museum. This consists of two stones running in contact, one being caused to traverse longitudinally, with a very slow motion. In this manner each stone corrects the defects of the other, and both are maintained accurately cylindrical in form.'