It's a set of dividers, and they're not just used aboard sailing ships, but were used well into the 20th Century, and are still used today. They're used to measure the distance between two points on a chart. One leg is placed at one point, usually the ship's position. The dividers are opened so the other leg is on the point you wish to measure to. The dividers can then be compared to the scale on the chart. The dividers can also be 'walked', leg over leg, to plan a journey, or to measure a distance that would require the dividers to be opened greater than practical.