This simple brass instrument was designed to aid chart work. According to the instructions, it was meant as a replacement for parallel rulers, in particular in order to avoid the wear and tear caused by constantly sliding them around the chart. It is held in a leather case, now in poor condition, with a printed sheet of instructions, which indicate that it was originally sold for 30 shillings.

The instrument itself is made of a circular disc that rotates in a metal holder. To use it, the straight edge is placed on the chart between the departure point and the destination, and then moved until the nearest meridian line can be seen through the hole in the centre of the milled wheel. The wheel is then turned until the north point of the disc aligns with the meridian line. The true course is shown by the index line at the side of the instrument.