As time passed, small improvements were made in watches. Brass replaced iron in the making of wheels and screws. Glass crystal was used to protect the faces. However, more than one hundred and fifty years watches were regarded as jewellery rather than timekeepers. Cases were made of rock crystal, silver, agate and gold. They were shaped like butterflies, shells, insects, crosses, or anything else an ingenious artisan could fashion. Mary, Queen of Scots, even had a watch shaped like a skull and the hinged jaw opened to show the dial.
Then in 1685 a major advance in watch making occurred. Robert Hooke, who has invented the anchor escapement for clocks, found that a vibrating spring would provide a regular rhythm for a watch just as a pendulum did for a clock. He made spring from a hog’s bristle, because it was tough enough to coil and uncoil again and again without breaking. For this reason, it was called a hairspring. It has kept this name although bristles have long since been replaced by metal springs.
The hairspring brought increased accuracy to watches. Shortly after its investing watchmakers began putting minute hands on their timepieces. Until then watches have been so inaccurate, even as to the hour, that it was not uncommon for a man to carry many as three in order to check one against another. There had been no use at all bothering with minutes.