says:  “To Halley we owe the first appreciation of the real complexity of the subject of magnetism.  It is wonderful indeed, and a striking proof of the penetration and sagacity of this extraordinary man, that with his means of information he should have been able to draw such conclusions, and to take so large and comprehensive a view of the subject as he appears to have done.”  In 1692, Halley explained his theory of terrestrial magnetism, and begged captains of ships to take observations of the variations of the compass in all parts of the world, and to communicate them to the Royal Society, “in order that all the facts may be readily available to those who are hereafter to complete this difficult and complicated subject.”

The extent to which Halley was in advance of his contemporaries, in the study of terrestrial magnetism, may be judged from the fact that the subject was scarcely touched after his time till the year 1811.  The interest which he felt in it was not of a merely theoretical kind, nor was it one which could be cultivated in an easy-chair.  Like all true investigators, he longed to submit his theory to the test of experiment, and for that purpose Halley determined to observe the magnetic variation for himself.  He procured from King William III. the command of a vessel called the “Paramour Pink,” with which he started for the South Seas in 1694.  This particular enterprise was not, however, successful; for, on crossing the line, some of his men fell sick and one of his lieutenants mutinied, so that he was obliged to return the following year with his mission unaccomplished.  The government cashiered the lieutenant, and Halley having procured a second smaller vessel to accompany the “Paramour Pink,” started once more in September, 1699.  He traversed the Atlantic to the 52nd degree of southern latitude, beyond which his further advance was stopped.  “In these latitudes,” he writes to say, “we fell in with great islands of ice of so incredible height and magnitude, that I scarce dare write my thoughts of it.”

On his return in 1700, Halley published a general chart, showing the variation of the compass at the different places which he had visited.  On these charts he set down lines connecting those localities at which the magnetic variation was identical.  He thus set an example of the graphic representation of large masses of complex facts, in such a manner as to appeal at once to the eye, a method of which we make many applications in the present day.

But probably the greatest service which Halley ever rendered to human knowledge was the share in which he took in bringing Newton’s “Principia” before the world.  In fact, as Dr. Glaisher, writing in 1888, has truly remarked, “but for Halley the ‘Principia’ would not have existed.”