A Letter to the Right Honourable the Earl of Macclesfield, President, the Council, and Fellows, of the Royal Society, concerning the Variation of the Magnetic Needle; With a Sett of Tables Annexed, Which Exhibit the Result of Upwards of Fifty Thousand Observations, in Six Periodic Reviews, from the Year 1700 to the Year 1756, Both Inclusive; And are Adapted to Every Five Degrees of Latitude and Longitude in the More Frequented Oceans. By William Mountaine and James Dodson, Fellows of the Royal Society