Halley's Chart of Magnetic Declinations

Abstract

WITHIN the last few days I have come into possession of another early map showing Halley's lines. The date of this map is 1725, and it was published by John Senex, F.R.S. It is entitled “A Map of the World, corrected from the Observations communicated to the Royal Societys of London and Paris.” The map consists of the two hemispheres, each of which is 21 inches in diameter. Around the margin in small print is Sir Isaac Newton's “Theory of the Tides,” and “An attempt to Assigne the Physical Cause of the Trade Winds and Monsoons, by Dr. Ed. Halley.” The map is particularly interesting, as it was evidently intended to give a full account of the winds, the directions of which in the trade winds and monsoons are indicated by arrows. Another interesting note in the margin is “Of the quantity of Vapour exhaled from the Sea, of its Circulation, and of the Cause of Springs,” “Extracted from a Discourse published in the Philosoph. Transact., No. 189, 192. Writ by Dr. Ed. Halley.” What makes the map so interesting is the notes printed upon it referring to the magnetic declinations. The lines of magnetic variation for every 5° east and west of the line of no variation, are given in the Atlantic and Indian Oceans, but not in the Pacific.