Contribution of Kepler’s Laws

The Principia consists of three books. The first and second, which occupy three-fourths of the work, are entitled On the Motion of Bodies, and the third bears the title On the System of the World.

Continuing Discovery of Gravity,
our selection from Memoirs of the Life, Writings and Discoveries of Sir Isaac Newton by Sir David Brewster published in 1855. The selection is presented in six easy 5 minute installments. For works benefiting from the latest research see the “More information” section at the bottom of these pages.

Previously in Discovery of Gravity.

Time: 1687
Place: Woolsthrope, England

In replying to this letter Dr. Halley assured him that Hooke’s “manner of claiming the discovery had been represented to him in worse colors than it ought, and that he neither made public application to the society for justice nor pretended that you had all from him.” The effect of this assurance was to make Newton regret that he had written the angry postscript to his letter; and in replying to Halley on July 14, 1686, he not only expresses his regret, but recounts the different new ideas which he had acquired from Hooke’s correspondence, and suggests it as the best method “of compromising the present dispute” to add a scholium in which Wren, Hooke, and Halley are acknowledged to have independently deduced the law of gravity from the second law of Kepler.

At the meeting of April 28th, at which the manuscript of the Principia was presented to the Royal Society, it was agreed that the printing of it should be referred to the council: that a letter of thanks should be written to its author; and at a meeting of the council on May 19th it was resolved that the manuscript should be printed at the society’s expense, and that Dr. Halley should superintend it while going through the press. These resolutions were communicated by Dr. Halley in a letter dated May 22d; and in Newton’s reply on June 20th, already mentioned, he makes the following observations:

The proof you sent me I like very well. I designed the whole to consist of three books; the second was finished last summer, being short, and only wants transcribing and drawing the cuts fairly. Some new propositions I have since thought on which I can as well let alone. The third wants the theory of comets. In autumn last I spent two months in calculation to no purpose, for want of a good method, which made me afterward return to the first book and enlarge it with diverse propositions, some relating to comets, others to other things found out last winter. The third I now design to suppress. Philosophy is such an impertinently litigious lady that a man had as good be engaged in lawsuits as have to do with her. I found it so formerly, and now I can no sooner come near her again but she gives me warning. The first two books, without the third, will not so well bear the title of Philosophies Naturalis Principia Mathematica; and therefore I had altered it to this: de Moti Corporum, Libri duo. But after second thoughts I retain the former title. ‘Twill help the sale of the book, which I ought not to diminish now ’tis yours.”

In replying to this letter on June 29th Dr. Halley regrets that our author’s tranquility should have been thus disturbed by envious rivals, and implores him in the name of the society not to suppress the third book. “I must again beg you,” says he, “not to let your resentments run so high as to deprive us of your third book, wherein your applications of your mathematical doctrine to the theory of comets, and several curious experiments which, as I guess by what you write ought to compose it, will undoubtedly render it acceptable to those who will call themselves philosophers without mathematics, which are much the greater number.”

To these solicitations Newton seems to have readily yielded. His second book was sent to the society, and presented on March 2, 1687. The third book was also transmitted, and presented on April 6th, and the whole work was completed and published in the month of May, 1687.

Such is the brief account of the publication of a work which is memorable not only in the annals of one science or of one country, but which will form an epoch in the history of the world, and will ever be regarded as the brightest page in the records of human reason. We shall endeavor to convey to the reader some idea of its contents, and of the brilliant discoveries which it disseminated over Europe.

The Principia consists of three books. The first and second, which occupy three-fourths of the work, are entitled On the Motion of Bodies, and the third bears the title On the System of the World. The two first books contain the mathematical principles of philosophy, namely, the laws and conditions of motions and forces; and they are illustrated with several philosophical scholia which treat of some of the most general and best-established points in philosophy, such as the density and resistance of bodies, spaces void of matter, and the motion of sound and light. The object of the third book is to deduce from these principles the constitution of the system of the world; and this book has been drawn up in as popular a style as possible, in order that it may be generally read.

The great discovery which characterizes the Principia is that of the principle of universal gravitation, as deduced from the motion of the moon, and from the three great facts or laws discovered by Kepler. This principle is: That every particle of matter is attracted by or gravitates to every other particle of matter, with a force inversely proportional to the squares of their distances. From the first law of Kepler, namely, the proportionality of the areas to the times of their revolution, Newton inferred that the force which kept the planet in its orbit was always directed to the sun; and from the second law of Kepler, that every planet moves in an ellipse with the sun in one of its foci, he drew the still more general inference that the force by which the planet moves round that focus varies inversely as the square of its distance from the focus. As this law was true in the motion of satellites round their primary planets Newton deduced the equality of gravity in all the heavenly bodies toward the sun, upon the supposition that they are equally distant from its center; and in the case of terrestrial bodies he succeeded in verifying this truth by numerous and accurate experiments.

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