Five-minute digest
On the Magnet (De Magnete)
Summary
De Magnete, magneticisque corporibus, et de magno magnete tellure (On the Magnet, Magnetick Bodies Also, and on the Great Magnet the Earth) was published in London in 1600 by William Gilbert, a Colchester-born physician who became president of the College of Physicians that year. It is the first large book devoted to experimental study of magnetism and one of the earliest works of natural philosophy built on systematic, repeated experiment. Gilbert's central claim is that the earth itself is a great magnet, and that the behaviour of the compass needle follows from this. The library text is the English translation made by members of the Gilbert Club and edited by Silvanus P. Thompson, printed at the Chiswick Press in 1900 to mark the book's tercentenary, with Thompson's extensive notes of 1901; the translation imitates the English of Gilbert's time. In the preface Gilbert addresses readers who seek knowledge not from books only but from things themselves, marks his own experiments with asterisks, and invites others to repeat them carefully. A commendatory preface by the mathematician Edward Wright praises the book's value to navigators. Book I reviews ancient and modern writers on the loadstone, dismissing fables such as the belief that garlic destroys its power, and describes loadstone and iron, their poles, and their medicinal uses. Its last chapter states the new doctrine that the globe of the earth is magnetic. Book II distinguishes magnetic attraction, which Gilbert calls coition, from the attraction of rubbed amber, jet and gems. Using a light pivoted needle, the versorium, he shows that many substances attract small bodies when rubbed, names them electrics, and argues that they act by an effluvium, while magnetism is a power of the whole substance. He then works with a terrella, a loadstone ground into a sphere, as a model of the earth, mapping its poles, meridians and equator. Book III treats direction and how iron acquires verticity when touched by a loadstone. Book IV explains the variation of the compass from true north as caused by the irregular distribution of the earth's land masses, and discusses whether longitude can be found from it. Book V treats the dip of the needle below the horizon, describes an instrument to find latitude from it, and ends with a chapter arguing that magnetic force is animate or imitates life. Book VI argues that the earth's magnetic nature explains its daily rotation on its axis, answers objections to the earth's motion, and discusses the precession of the equinoxes. Galileo admired the book, Kepler drew on it for his celestial physics, and Francis Bacon, while borrowing from it, criticized Gilbert for building a whole philosophy on the loadstone.
Top Lessons
Learn from things, not only from books
Gilbert addresses readers who seek knowledge from things themselves. He reviews earlier writers but tests their claims, rejecting the story that garlic robs a magnet of its power, and asks readers to repeat his experiments carefully rather than dismiss them when they fail.
Distinguish what looks alike
Amber and the loadstone both seem to attract, and earlier writers often treated them together. Gilbert shows with the versorium that many rubbed substances attract all light bodies, while the magnet acts only on magnetic bodies and without rubbing, and so he separates electric from magnetic attraction.
A model of the world in the hand
Gilbert's terrella, a loadstone shaped into a sphere, lets him study on a table the behaviour he believes the compass shows across the whole earth. Reasoning from a model to the world is one of his most important methods, and his conclusion that the earth is a magnet rests on the agreement between the two.
Honest medicine
As a physician, Gilbert examines the medical uses claimed for loadstone and iron. He rejects remedies built on fables, such as loadstone for headaches, warns against prescribing iron indiscriminately for every obstruction of the liver and spleen, and accepts only those uses he judges confirmed by experience.
Verified Quotes
“But to you alone, true philosophizers, honest men, who seek knowledge not from books only but from things themselves, have I addressed these magnetical principles in this new sort of Philosophizing.”
“for nothing hath been set down in these books which hath not been explored and many times performed and repeated amongst us.”
“To those early forefathers of philosophy, Aristotle, Theophrastus, Ptolemy, Hippocrates, and Galen, let due honour be ever paid: for by them wisdom hath been diffused to posterity; but our age hath detected and brought to light very many facts which they, were they now alive, would gladly have accepted.”
“The application of a loadstone for all sorts of headaches no more cures them (as some make out) than would an iron helmet or a steel cap.”
“we have to establish & present to the view of the learned our New & unheard of doctrine about the earth”
“make yourself a versorium of any metal you like, three or four digits in length, resting rather lightly on its point of support after the manner of a magnetick needle”
“A loadstone is a wonderful thing in very many experiments, and like a living creature.”
Medical Lens
Gilbert wrote as a practising London physician, and Book I, Chapters XIV and XV, are a short critical review of magnetic and chalybeate therapy. He reports ancient claims that loadstone expels gross humours or makes people melancholic, rejects loadstone as a cure for headaches or a means of drawing arrowheads from wounds, and condemns sellers who exploit its supposed dangers for gain. On iron he works within humoral medicine, recommending prepared steel for a relaxed, humid liver and an enlarged spleen and for pale, sickly girls, while warning that physicians who give it for every obstruction, dropsy, jaundice or hypochondriacal melancholia harm their patients. His insistence that medical claims be tested by experience parallels his method in physics. Read these chapters as a record of Elizabethan pharmacology, not as clinical guidance.
- Book I, Ch. XIV
Survey and criticism of medicinal claims for the loadstone: purging humours, causing melancholy, curing headaches, extracting arrowheads, and its use in wound plasters.
- Book I, Ch. XV
Preparation and use of iron and steel as remedies for disorders of the liver and spleen, with warnings against their misuse and a discussion of whether iron is warm or cold.
- Book II, Ch. II
Galen's three kinds of attraction and the analogy of the cupping-glass, which Gilbert rejects as explanations of amber's attraction.
- Edward Wright's preface
Wright's praise of Gilbert as a distinguished doctor of medicine who gave his intellect to magnetism as well as medicine.