This type of ruler would have been used by navigators to draw lines on sea charts.
This particular ruler is actually a special type of ruler for two reasons. First, it is what is called a 'parallel ruler' because as well as straight lines it could also be used to draw parallel lines. Parallel rulers are quite common because drawing parallel lines is such a common thing to need to do. There are different ways to design a ruler so that it can be used to draw parallel lines. One way is to put wheels at each end so you can push it along the paper. A better way is to fit it with a roller all the way along. Another way is to split the ruler in half and to join the two halves together with pivots. That is what has been done in this ruler.
© Museum of the History of Science, Oxford
The second special thing about this ruler is that is has two brass plates bolted together and fixed to it half way along one side. One of the plates is square and the other is round. You can see them in the picture. These metal plates are to do with using the ruler on sea charts. They would help a captain navigate a ship correctly by allowing him to draw his direction and position on the sea map accurately. The square one is labelled with the points of the compass, north, south, east and west, and has a scale of 360 degrees on it. The round plate is only labelled with degrees.
The two brass plates allow this ruler to be used to draw lines that are almost parallel to each other but not quite. This is useful because of a thing called 'magnetic declination', which is a variation between the real and magnetic poles. This magnetic variation is to do with the use of a magnetic compass. Normally when we use a magnetic compass, such as when we are out walking and using it to find the direction we are going in, we think that the needle points north, towards the North Pole. This is not quite true. The needle doesn't point exactly to the North Pole. It actually points to what is called the Magnetic North Pole. This is not the same as the real North Pole, or what is called the Geographic North Pole. The Geographic North Pole is a fixed place on the earth corresponding to the point that the earth seems to rotate around. The Geographic North Pole never changes position. The Magnetic North Pole on the other hand isn't exactly at the Geographic North Pole. It is the imaginary point that a needle on a compass points to. It changes its position slowly all the time because of changes in the earth's magnetic field.
When at sea, captains of ships needs to know how far the real North Pole is away from Magnetic North Pole. This is so that they can steer correctly by using a magnetic compass. The metal plates on this ruler allow the ruler to be set up so that the difference in the position of the real North Pole and the Magnetic North Pole is automatically taken into account when drawing lines with it.
Throughout history, navigating ships has been one of the most important practical uses of mathematics. Many mathematicians have spent a lot of time thinking up theories and designing mathematical instruments to help with navigation. Captains of ships have always had to be very good at maths otherwise they would lose their position and be in danger of crashing their ships into rocks. They also need to be able to use lots of different types of mathematical instruments, like this special type of ruler, as well as things like sextants.
One famous navigator who got his sums wrong was called Admiral Sir Clowdisley Shovell. He was head of the English Fleet that had been to Gibraltar to fight the French. When he was returning home to England he got the position of his ships so wrong that four out of five of them were smashed to pieces on the rocks of the Scilly Isles off the coast of Cornwall. Hundreds of his sailors were drowned. Sir Clowdisley, however, was washed up onto a beach and survived.