The Memorial of Pilot

Respectfully represents.

The he having observed in the public papers of the present Year, a proposal of a premium to be annually disposed of, from a donation of a Lover of useful Knowledge in London, to be adjudged by the Society to the author of the best discovery, or most useful invention relating to Navigation, Astronomy, or natural Philosophy; and he being solicitous as well for the prosperity of Science, as to offer himself a Candidate for the said premium, for the present Year 1787, he hereby presents the Society with a description of the Variation Globe, notwithstanding a Theory of the Variation of the Compass has been lately attempted, he apprehends the description of the Variation Globe has not been made public which is as follows vizt.

A Magnetic Meridian for any part of this Globe is a Circle drawn through the place of observation and the two Magnetic poles. Only one of these Magnetic Meridians divide the Earth in two equal parts which one is of consequence a Great Circle.

In that Magnetic Meridian which is a Great Circle a point is fixed which is at equal distance from the two Magnetic Poles. From the said Point a Great Circle is drawn at right Angles with that Magnetic Meridian which is a Great Circle, which will be the Magnetic Equator.

In different parts of the Magnetic Equator are the Centres from which all those lesser circles are drawn representing the magnetic Meridians.

The angles between the Magnetic Meridians (drawn after this Manner) and the true Meridians are the Variation of the Compass which are found Universally to correspond with the observations made by the celebrated Captain Cook in his last Voyage.

If the two Magnetic Poles were diametrically opposite to each other all the Magnetic Meridians would be Great Circles and the Line of no variation would coincide with the Terrestrial Meridian but this is found not to be the case.

After knowing the Variation of the Compass at different places on the Globe at the same time, where the Latitude Longitude and Variation are known, we are enabled to find the places of the Magnetic Poles for that time: in the year 1777 the Northern one was in Latitude 76°. 4' N and Longitude 86°. 45' West from Greenwich and the Southern one in the same Year was in Latitude 72° South and Longitude 140' East from the same place.

By knowing the variation for different times and different places we are able to determine the periods of Revolution of the Magnetic Poles, which are found to Revolve from West to East.

The places and periods of Revolution of these Poles being once known we are able to find the Variation of the Compass Geometrically by the Variation Globe, for any time past present or to come, hence we find that a problem determined by the Globe Geometrically may also be performed by Calculation.