He also, from observations of the sun's altitude, computed the difference of latitude of the two places, and so calculated the length of a degree of the meridian.

The apparent difference of latitude between two stations on opposite sides of the mountain were compared with the real difference of latitude obtained by triangulation.

He then combines, by addition and substraction, all the differences of latitude and departure, to get the overall difference of latitude and departure, and converts that back to overall bearing and distance made good.

Difference of latitude and departure (or longitude) are two sides of the triangle forming a right angle, the distance travelled is the hypotenuse, and the angle between difference of latitude and distance is the course.

In 1772 was published in Dublin his 'Tables of Difference of Latitude and Departure for Navigators, Land Surveyors, &c.,' in which he recommends that in surveying the bearing of objects should be taken from the meridian of the place.

In the west the flora resembles that of central Europe, while in the east it is distinctly Mediterranean in character, though the difference of latitude is only about 1 , on both sides of the chain from the centre whence the Corbières stretch north-eastwards towards the central plateau of France.