own motion but at the instance of his friend, Halley, who subsequently boasted that he was the Ulysses who had discovered Achilles and brought him forth from his concealment. In the month of August, 1684, Halley, having become satisfied that Hooke could not furnish the demonstration of the law of gravitation which he had repeatedly promised, visited Cambridge to confer with Newton on the subject on which he had become deeply interested and been long laboring without any satisfactory result. He inquired of Newton, what would be the curve described by the planets on the supposition that the attractive influence of the sun diminished as the square of the distance? Newton at once replied, 'An ellipse.' When asked how he knew this, he replied: 'I have calculated it.' Halley, surprised and delighted at the announcement, asked to see the demonstration. Newton was unable to lay his hands on the calculation he had made two years before, nor could he at the moment reproduce it. He promised Halley however that he would send him the demonstration as soon as he was able, and in the month of November following he fulfilled his promise. Halley immediately revisited Cambridge to obtain Newton's consent to the publication of the discovery. In this he succeeded, and on the 10th of December he informed the Royal Society of the discovery and that Newton had consented to prepare a paper on the subject for the society. In February, 1685, the promised communication was received—a paper of twenty-four pages, containing four theorems and seven problems. He refers to it in the accompanying letter, as his 'Notions about Motion.' This humble yet most memorable paper ever presented to the Society was the germ of 'The Principia.'
The great discovery having been made public, Newton seems to have felt that the time had come to enter on the gigantic task he had doubtless proposed to himself when the discovery was first made but which other occupations had hitherto prevented him undertaking, namely, putting his demonstration in a complete and conclusive form and applying it to the solution of the many interesting and sublime problems which the phenomena of the material universe presented. For two years he dismissed from his mind all other occupation, and devoted himself with all the energy of his mighty intellect to the Herculean task. With untiring industry, prolonged attention, and intense thought, probably never paralleled in the history of intellectual effort, he lived but to meditate and calculate; oftentimes so wholly absorbed with the grand themes which occupied his mind as to be for the time unconscious of all the ordinary concerns of life. Frequently, on rising in the morning he would sit for hours on his bedside arrested by some new conception, and had it not been for the attention of the members of his family would often have neglected to take his daily food.
