Book IV
The Fourth Book treats of the variation at different places; says that it is due to inequality among the earth’s elevations;[35] shows that variation and direction are due to the controlling force of the earth and the rotatory magnetic nature, not by an attraction or a coition or by other occult cause; explains the different modes of constructing the mariner’s compass, in vogue at the time,[36] and how the deviation of the needle is greater or less according to the distance of place.
Book V
In this Fifth Book is to be found everything relative to the dip of the magnetic needle, likewise the description of an instrument for showing, by the action of a loadstone, the degree of dip below the horizon in any latitude; and the announcement that the magnetic force is animate or imitates a soul; in many respects, it surpasses the human soul while that is united to an organic body.
Book VI
Throughout this last Book, Gilbert glories in the Copernican theory, the open, unquestioned, advocacy and endorsement of which according to many seems, after all, to have been the object of the work. He maintains that the magnetic axis of the earth remains invariable; he treats of the daily magnetic revolution of the globes, as against the time-honoured opinion of a primum mobile, the fixed stars being at different distances from the earth; of the circular motion of the earth and of its primary magnetic nature, whereby her poles are made different from the poles of the ecliptic, as well as of the precession of the equinoxes and of the obliquity of the zodiac.
According to Humboldt,[37] Gilbert was the first to make use of the words electric force, electric emanations, electric attraction, but, he says, there is not found in “De Magnete” either the abstract expression electricitas or the barbarous word magnetismus introduced in the seventeenth century. We likewise owe to Gilbert the words equator, magneticum, terrella, versorium and verticitas, but not the word pole, which had before been used by P. Peregrinus and others.
The second edition of “De Magnete” appeared at Stettin in 1628, “embellished with a curious title-page in the form of a monument ... and a fantastic indication of the earliest European mariner’s compass, a floated lodestone, but floating in a bowl on the sea and left behind by the ship sailing away from it.”[38]
The third edition was also published at Stettin during 1633. Gilbert left, besides, a posthumous work, “De Mundo Nostro Sublunari Philosophia Nova,” Amsterdam, 1651, which latter, says Prof. Robison, consists of an attempt to establish a new system of natural philosophy upon the ruins of the Aristotelian doctrine.[39]
To give here such an analysis as Gilbert’s admirable work merits would be impracticable, but the short review of it made by Prof. Robison (at p. 209 of his “System of Mechanical Philosophy,” London, 1822) deserves full reproduction, as follows: “In the introduction, he recounts all the knowledge of the ancients on the subject treated, and their supine inattention to what was so entirely in their hands, and the impossibility of ever adding to the stock of useful knowledge, so long as men imagined themselves to be philosophizing, while they were only repeating a few cant words and the unmeaning phrases of the Aristotelian school. It is curious to mark the almost perfect sameness of Dr. Gilbert’s sentiments and language with those of Lord Bacon. They both charge, in a peremptory manner, all those who pretend to inform others, to give over their dialectic labours, which are nothing but ringing changes on a few trite truths, and many unfounded conjectures, and immediately to betake themselves to experiment. He has pursued this method on the subject of magnetism, with wonderful ardour, and with equal genius and success; for Dr. Gilbert was possessed both of great ingenuity, and a mind fitted for general views of things. The work contains a prodigious number and variety of experiments and observations, collected with sagacity from the writings of others, and instituted by himself with considerable expense and labour. It would, indeed, be a miracle if all of Dr. Gilbert’s general inferences were just, or all his experiments accurate. It was untrodden ground. But, on the whole, this performance contains more real information than any writing of the age in which he lived, and is scarcely exceeded by any that has appeared since. We may hold it with justice as the first fruits of the Baconian or experimental philosophy.” Elsewhere, Prof. Robison remarks: “It is not saying too much of this work to affirm that it contains almost everything we know of magnetism. His unwearied diligence in searching every writing on the subject and in getting information from navigators, and his incessant occupation in experiments, have left very few facts unknown to him. We meet with many things in the writings of posterior inquirers, some of them of high reputation and of the present day, which are published and received as notable discoveries, but are contained in the rich collection of Dr. Gilbert.”