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Show [FOCUS:~. CENTURfUM ISAAC N WTON (continued from page 10) several lesser works, there exists an astonishing amount of material on s ubjects relati ng to the Bible. The King James Version of th e Bible had appeared as recentl y as 16 11 , and Newton was an avid student of it all his life. He wrote extensively about m atters that interested him in the sphere of religion. It has been suggested by several scholars that one of the reasons for his fai lure to publish this m ateri al is that his religious views were not q uite orthodox-parti cularly in regard to the nature of the Trinity. e had a great deal to say about the prophecies of Daniel and St. John, and a book containing his views on these subjects came o ut after his death. A short work dealing with corruptions of the scriptures was published long after his deat h; but one work, titled the Chronology of the Ancient Kingdoms Amended, appeared the year after his death. In addition to his work on religion and history, he wrote extensively on chemistry and alchem y, none of it apparently intended for publication . But none of this was kn own even by the scientific public, w ho knew Newton chiefl y by his two stunning publications, the Principia and th e Opticks, the one appearing in Latin in 1687, the other in English in 1704. Had the un publ ished m aterial appeared in pri nt-about one and-a-half-million wordsit wou ld have fi lled about 25 standard size volumes. Now, if one were to pick a time that best expressed Newton's genius, it wou ld be much earl ier than the publication dates of his two masterpi eces of scientific writing; it would be the years 1665- 1666. In English history, 1665-1666 is call ed the an nus mi rabilis, the Marvelous Yea r, or the year of wonders; and it took that name fro m three separate traumatic events which struck London almost simultaneo usly: The Buboni c Plague, whi ch started late in 1664, and was the worst outbreak since the days of the Black Death in the 14th century, had in onl y two mo nths in the summer of 1665 wiped out one- J 0th of th e population of London. The Great Fire of London which decimared the o lder city that same yea r was credited with stemming the plague, but the city was left in smoldering ruins. As if that were not enough, because England was at war with Holland, the Dutch fl eet sailed up the Thames River and threatened London. By 1665, the plague was so pervasive that Cambridge was closed, and Newton,·who had just taken his bachelor's degree, was forced to return to his home in Woolsthorpe. The eighteen months he spent there waiting fo r Cambridge to reopen were the most eventful months of his life from the perspecti ve of science-his own annus mi rabi lis. During that period, even w hile he continued his experiments in chemistry and alchem y, often fashioning the m aterials he needed for his experiments, he worked out the basic laws of m echanics (his three laws of m otion), the principle of gravitation, his infinitesim al calculus, and his views on light and color-all this he did while still in his twen ties! Perhaps it was inevitable-that someone subscribing to the idea that the entire universe could be described simply as matter in motion, wou ld be com pelled to define th e laws that governed that motion-assuming it was not all random-or there would be no way of accounting fo r the mechanical universe. For Newton, then, the laws of motio n were a good starting point. The first axiom reads: " Every body perseveres in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by impressed forces." The second axiom reads: "Change of m otion (i.e. rate of change of momentum) is proportional to the impressed force and takes place in the direction in which that force is impressed." And the third axiom reads: "To every action there is always opposed an equal reaction; o r the mutual actions of two bodies upon each other are always equal and opposite." Actually, the laws of m otion were not strictly new . Descartes and Galileo had contributed substantially to the first two, and Newton credits them for doing so. But N ewton was the first to state them unequivocally as laws, add the third law, and link the three laws together. Scholars have pointed out contributions to the first two laws by fourteenth century thinkers, antedating both Galileo and Kepler; but they assert that w hile Newton drew from the experiments of Wallis, Wren, and Hu ygens for the third law- the onl y physical law of the three-no one before N ewton clearly formulated it. But Leonardo da Vinci had done so a h undred and fifty years earlier, using language almost identical to Newton's. Although the ideas that Newton so firmly grasped during those fantastically productive eighteen months at Woolsthorpe would not be presented to the world until 1687, practically all of the ideas contained in the Principia were born in his home town; they were the product of a mind that penetrated seemingly inscrutable problems until they yielded all their secrets. The laws of m otion were the product of experience and observation -his and others. But he took them one step further. With his theory of gravitation, he was careful not to assign the cause of motion. He did not presume to know it. But there were a few things he did know with all the certainty of his splendidly intuitive mind, supported by the mathematical formula he had invented to explain it. The story of the apple falling at his feet may be a myth, but he may well have wondered if the force that attracts the apple to the earth reaches out to attract the moon, and if that force is what keeps the moon in its o rbit. Some force was necessary to keep a circular body in orbit, a centripetal force, he reasoned, or, like the apple, it would fall to earth. So, the force that kept the I moon circling the earth must be the same force that keeps the earth in its orbit about the sun. All matter in motion was affected by that force. But centripetal is not one force and gravity another. They are the sam e force. The speeds that planets traveled in their orbits, Newton calculated, was a function of their distance from the sun; as their distances increased, their speeds decreased. Then he supplied the law he had learned from Kepler while a student at Cambridge: force, like speed, must decrease with distance. The law is known as the 'inverse square law.' It requires that a planet twice as far away would register a force one quarter as great; a planet three times as far would register a force one ninth as great, etc. Newton registers his discovery in a manuscript which helps to establish the importance of his annus mirabilis: 11 And the same year [ l 666] I began to think of gravity extending to the Orb of the Moon, and having fo und out how to estimate the force with which [a] globe revolving within ;i sphere presses the surface of the sphere from Kepler's Rule of the periodical times of the Planets being in a sesquialterate proportion of their distances from the centres of their Orbs I deduced that the forces w hich keep the Planets in their Orbs must [be] reciprocally as the squares of their distances from the centres about which they revolve; and thereby compared the force requisite to keep the Moon in her Orb with the force of gravity at the surface of the Earth, and fo und them answer pretty nearly." ut, when Newton derived from this his aw of universal gravitation, he, like his contemporaries, believe9 that all orbits of celestial bodies were circular. He was soon to discover that the required orbit under an inverse square law is an ellipse. When the Principia was published in 1687, it appeared in three books. In the first book, Newton sets forth some stunning conclusions. For example, if the sun exerts a force on the planets, the planets exert an equal force on the sun (from the third law or axiom ). T he apple pulls as hard on the earth as the earth pulls on the app le; but in this case, just as in the case of the respective pull of the sun and its planets, the m ass is significantly greater in one body than in the other. Newton goes on to explain that gravity acts as if all the mass were at the center of the sphere, yet he was to discover that inside the sphere the rule does not apply. As the center is approached, gravity decreases; at th e center, gravity is zero. T he force of gravity from the outer parts of the sphere cancel each other. He goes on to show that pull is based on mass or weight. If the mass is doubled, the pull will be doubled. If the moon were half the mass it is, the pull would be half and the centripetal force would retain the balance between the moon and the earth at the same distance. In Book II of Principia, Newton continues his examination of motion; but while Book I deals with motion through media of no or little resistance, Book IT deals with motion through media offering resistance, like air or water. Among his investigations in this connection is his determination of the speed of sound. Book ID is a report of applications of the theories contained in Books I and II. It includes his conclusion regarding the shape of the earth, not a perfect sphere as formerly believed, but bulging at the equator because of its rotation on its axis. Also included is a report on the tides, caused by the joint attraction of the moon and the (continued on page 12) .,. |