Enclosure

The Memorial of John Churchman respectfully represents

That the Variation of the Compass and its hitherto imagined uncertain Laws have long engaged the attention of Philosophers And Mathematicians. Why it should move at one Time slow, at another Time quick, now become stationary and then retrograde, has puzzled the Enquirer, and although these varieties have been continually Subjects of observation, Yet I have never heard that any Regular System hath hitherto been published to account for or foretel upon any rational plan, what will be the future Movements of this wonderful Phenomenon, the Magnetic Influence. Whether My attempts to reconcile all these Difficulties and of consequence to build on them a certain System of Longitude will prove true or false must be left to the world to judge.

From a variety of observations, Reflections and Deductions, the following Conclusions are assumed.

• 1st. That the Magnetic Needle hath a Direction to two Points at certain Distances, one from the North, the other from the South Pole of the Earth.
• 2ly. That these Points, to which it hath a Direction are properly called Magnetic Poles.
• 3ly. That the Magnetic Poles perform Revolutions in certain given Times from West to East.
• 4ly. That by a Variety of Deductions, and by laying down Many observations of the variation of the Magnetic Needle, one of these Poles is found to be at a certain Distance from the North Pole of the Earth, which for distinction May be called the North Magnetic Pole.
• 5ly. That the orbit in which the South Magnetic Pole moves, is larger than the Northern orbit.
• 6ly. That the angle between a Meridian of the Earth and the Magnetic Meridian is the Variation of the Compass.
• 7ly. That the Northern Magnetic Pole in 1779 was in Latitude 76.° 4’ and Longitude 85.° 12’ west from London and its period of Revolution is 463 years 344 days.
• 8ly. That the Situation of the Southern Magnetic Pole for want of a sufficient Number of observations, I have not yet so accurately determined, but in the Year 1777 I calculate it to have been in 72 degrees South Latitude and 140 degrees East Longitude from London or Greenwich.

That from hence the Longitude of either Pole is easily determined, and I have formed a Sett of Tables of their Annual Situations from the Year 1657 (when a Line of no variation passed over London) to the Year 1888, when a Line of no Variation will again pass the same Place.

That having the Latitude of the Magnetic Poles and by ascertaining the Longitude of the same from the Tables for any given Time, we are able to determine the Longitude of any Place with the greatest Precision from a combined observation of the Latitude of the Place and the Variation of the Magnetic Needle.

It may be necessary to observe that a Magnetic Meridian for any part of this Globe is a circle drawn through the two Magnetic Poles and the place of observation. If the Magnetic Poles were diametrically opposite to each other, the Line of no Variation would coincide with the Meridian of the Earth, and all the Magnetic Meridians would of consequence be great Circles; but though this is not the case, the Longitude is determined by a Method full as simple as if they were great Circles; that the Magnetic Meridians meet in the two Magnetic Poles, and that these Poles are in certain parts of their orbits in certain Times. There is sufficient Proof, by laying down on a Globe the observations of the Variation Made by the late celebrated Cap. Cook in his last Voyage round the world as well as those made by other able Navigators.