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Writing and publication

In November 1684, Halley received a treatise of nine pages from Newton called De motu corporum in gyrum (On the motion of bodies in an orbit). It derived the three laws of Kepler assuming an inverse square law of force, and generalized the answer to conic sections. It extended the methodology of dynamics by adding the solution of a problem on the motion of a body through a resisting medium. After another visit to Newton, Halley reported these results to the Royal Society on December 10, 1684 (Julian calendar). Newton also communicated the results to Flamsteed, but insisted on revising the manuscript. These crucial revisions, especially concerning the notion of inertia, slowly developed over the next year-and-a-half into the Principia. Flamsteed's collaboration in supplying regular observational data on the planets was very helpful during this period.

The text of the first of the three books was presented to the Royal Society at the close of April, 1686. Hooke's priority claims caused some delay in acceptance, but Samuel Pepys, as President, was authorised on 30 June to license it for publication. Unfortunately the Society had just spent their book budget on a history of fish, so the initial cost of publication was borne by Edmund Halley. [1] The third book was finally completed a year later in April, 1687, and published that summer.

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The contents of the book

In the preface of the Principia, Newton wrote6

... rational mechanics will be the science of motion resulting from any forces whatsoever, and of the forces required to produce any motion ... and therefore I offer this work as the mathematical principles of philosophy, for the whole burden of philosophy seems to consist in this — from the phenomena of motions to investigate the forces of nature, and then from these forces to demonstrate the other phenomena ...

It was perhaps the force of the Principia, which explained so many different things about the natural world with such economy, that caused this method to become synonymous with physics, even as it is practiced almost three and a half centuries after its beginning. Today the two aspects that Newton outlined would be called analysis and synthesis.

The Principia consists of three books

1. De motu corporum (On the motion of bodies) is a mathematical exposition of calculus followed by statements of basic dynamical definitions and the primary deductions based on these. It also contains propositions and proofs that have little to do with dynamics but demonstrate the kinds of problems that can be solved using calculus.
2. The first book was divided into a second volume because of its length. It contains sundry applications such as motion through a resistive medium, a derivation of the shape of least resistance, a derivation of the speed of sound and accounts of experimental tests of the result.
3. De mundi systemate (On the system of the world) is an essay on universal gravitation that builds upon the propositions of the previous books and applies them to the motions observed in the solar system — the regularities and the irregularities of the orbit of the moon, the derivations of Kepler's laws, applications to the motion of Jupiter's moons, to comets and tides (much of the data came from John Flamsteed). It also considers the harmonic oscillator in three dimensions, and motion in arbitrary force laws.

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The sequence of definitions used in setting up dynamics in the Principia is exactly the same as in all textbooks today. Newton first set out the definition of mass6

The quantity of matter is that which arises conjointly from its density and magnitude. A body twice as dense in double the space is quadruple in quantity. This quantity I designate by the name of body or of mass.

This was then used to define the "quantity of motion" (today called momentum), and the principle of inertia in which mass replaces the previous Cartesian notion of intrinsic force. This then set the stage for the introduction of forces through the change in momentum of a body. Curiously, for today's readers, the exposition looks dimensionally incorrect, since Newton does not introduce the dimension of time in rates of changes of quantities.

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He defined space and time "not as they are well known to all". Instead, he defined "true" time and space as "absolute" and explained:

Only I must observe, that the vulgar conceive those quantities under no other notions but from the relation they bear to perceptible objects. And it will be convenient to distinguish them into absolute and relative, true and apparent, mathematical and common. [...] instead of absolute places and motions, we use relative ones; and that without any inconvenience in common affairs; but in philosophical discussions, we ought to step back from our senses, and consider things themselves, distinct from what are only perceptible measures of them.

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It is interesting that several dynamical quantities that were used in the book (such as angular momentum) were not given names. The dynamics of the first two books was so self-evidently consistent that it was immediately accepted; for example, Locke asked Huygens whether he could trust the mathematical proofs, and was assured about their correctness.

However, the concept of an attractive force acting at a distance received a cooler response. In his notes, Newton wrote that the inverse square law arose naturally due to the structure of matter. However, he retracted this sentence in the published version, where he stated that the motion of planets is consistent with an inverse square law, but refused to speculate on the origin of the law. Huygens and Leibniz noted that the law was incompatible with the notion of the aether. From a Cartesian point of view, therefore, this was a faulty theory. Newton's defence has been adopted since by many famous physicists — he pointed out that the mathematical form of the theory had to be correct since it explained the data, and he refused to speculate further on the basic nature of gravity. The sheer mass of phenomena that could be organised by the theory was so impressive that younger "philosophers" soon adopted the methods and language of the Principia.

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The mathematical language

The reason for Newton's use of Euclidean geometry as the mathematical language of choice in Principia is puzzling in two respects. The first is the trouble that today's physicists, trained in modern analytical methods, face in following the arguments. This mathematical language reportedly baffled Richard Feynman to the extent that he tried to work out alternative Euclidean proofs to his own satisfaction. S. Chandrasekhar, in one of his last major efforts, translated the Principia into modern mathematical language so that physicists of today can read and appreciate the book that founded modern physics.

The second puzzle is historical. Why did Newton revert to Euclidean methods, when seventeenth century mathematics increasingly used Descartes' analytical geometry for its transactions? Newton himself had written earlier tracts using this language. Even his earlier communications on the calculus of differentials referred to a new language of fluxions that he had invented. In fact, his early notebooks suggest strongly that he learnt Cartesian geometry long before he came to Euclid. Some commentators have suggested that Newton used the mathematical language of Euclid in order to make a rhetorical point about how his methods followed easily from the Greek tradition. However, this piece of rhetoric was unnecessary for his contemporaries, who knew very well the general nature of Newton's mathematical discoveries.

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Location of copies

Several national rare-book collections contain original copies of Newton's Principia Mathematica, including:

* The Wren Library in Trinity College, Cambridge, has Newton's own copy of the first edition, with handwritten notes for the second edition.
* The Whipple Museum of the History of Science in Cambridge has a first-edition copy which had belonged to Robert Hooke.
* Fisher Library in the University of Sydney has a first-edition copy, annotated by a mathematician of uncertain identity and corresponding notes from Newton himself.
* The Pepys Library in Magdalene College, Cambridge, has Samuel Pepys' copy of the third edition.
* The Martin Bodmer Library[2] keeps a copy of the original edition that was owned by Leibniz. In it, we can see handwritten notes by Leibniz, in particular concerning the controversy of who invented calculus (although he published it later, Newton argued that he developed it earlier). As an interesting side note, the copy shows clear signs that Leibniz was an avid smoker.[citation needed]
* A first edition is also located in the archives of the library at the Georgia Institute of Technology. The Georgia Tech library is also home to a second and third edition.

A facsimile edition was published in 1972 by Alexandre Koyré and I. Bernard Cohen, Cambridge University Press, 1972, ISBN 0-674-66475-2.

Two more editions were published during Newton's lifetime:

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Second edition

Richard Bentley, master of Trinity College, influenced Roger Cotes, Plumian professor of astronomy at Trinity, to undertake the editorship of the second edition. Newton did not intend to start any re-write of the Principia until 17091. Under the weight of Cotes' efforts, but impeded by priority disputes between Newton and Leibniz2, and by troubles at the Mint3, Cotes was able to announce publication to Newton on 30 June 17134. Bentley sent Newton only six presentation copies; Cotes was unpaid; Newton omitted any acknowledgement to Cotes.

Among those who gave Newton corrections for the Second Edition were:

* Firmin Abauzit
* Roger Cotes
* David Gregory

However, Newton omitted acknowledgements to some because of the priority disputes. John Flamsteed, the Astronomer Royal, suffered this especially.

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Third edition

The third edition was published 25 March 1726, under the stewardship of Henry Pemberton, M.D., a man of the greatest skill in these matters ...; Pemberton later said that this recognition was worth more to him than the two hundred guinea award from Newton.5

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Ok, nearly there now...