is aligned with the sun when the shadow of the thread falls on the vertical bar. Yet another design has a magnifying glass on one vane that focuses the rays of the sun on the opposite vane. Meridional azimuth compasses included a universal equatorial sundial. By setting the date and latitude, the true north and the magnetic variation could be read from the instrument without the need for calculation. With the iron vessels introduced in the 19th century the azimuth compass would be mounted on a pedestal at a neutral point where the readings would not be affected by magnetism of the vessel. Applications Early navigators in the northern hemisphere could calculate latitude relatively easily when the night sky was clear by observing the elevation of Polaris, a star that is very close to the north celestial pole. However, calculating longitude was impossible until chronometers that could keep time accurately throughout a long voyage were developed. In 1588 King Philip II of Spain (1556–1598) offered a large reward to anyone who could find a practical solution to determining longitude. A novel approach, apparently originating with the Jesuit missionary Christoforo Borri, was to create charts that mapped points of equal magnetic declination. With an accurate reading of the latitude and the magnetic declination the navigator could determine their longitude using the chart. A drawback to this approach is that magnetic variations change over time, so the charts would need constant revision. The azimuth compass still had great value in letting the master of a ship determine how far the magnetic compass varied from true north, so he could set a more accurate course while following a line of constant latitude or using dead reckoning to navigate. In 1795 a British First Rate ship would have up to eight compasses, of which one was an improved steering compass. This azimuth compass may have been specialized for the purpose of measuring magnetic variation through taking readings from stars, and used to determine the accuracy of the other compasses. Instruments An azimuthal compass was described in 1736, but the inventor was not named. Azimuth compasses were sometimes large, with a brass case mounted in gimbals containing the rose, and sights on top of the case. Walter Hayes, Richard Glynne and Benjamin Ayres (died c. 1775) made accurate large azimuth compasses. Henry Gregory (1744–1782) had an establishment known as "The Azimuth Compass" in Leadenhall Street,