matter attracts every other particle, and suspected that the attraction varied as the product of their masses, and inversely as the square of the distance between them; but it is certain that he did not then know what the attraction of a spherical mass on any external point would be, and did not think it likely that a particle would be attracted by the earth as if the latter were concentrated into a single particle at its centre. Hence he must have thought that his discoveries of 1679 were only approximately true when applied to the solar system. His mathematical analysis, however, now showed that the sun and planets, regarded as spheres, exerted their attractions as if their masses were collected at their centres; and thus his former results were absolutely true of the solar system, save only for a correction caused by the slight deviation of the sun, earth, and planets, from a perfectly spherical form. The first book of the Principia is given up to the consideration of the motion of particles or bodies in free space either in known orbits, or under the action of known forces, or under their mutual attraction. It is prefaced by an introduction on the science of Dynamics; it also contains geometrical investigations of various properties of conic sections. The second book treats of motion in a resisting medium. The theory of Hydrodynamics was here created, and it was applied to the phenomena of waves, tides, and acoustics. In the third book, the theorems of the first are applied to the chief phenomena of the solar system; and the masses and distances of the planets and (when sufficient data exist) of their satellites are determined. In particular, the motion of the moon, with its various inequalities, and the theory of the tides, are worked out in detail, and as fully as was then possible. Newton also investigated the theory of comets, showed that they belonged to the solar system, and illustrated his results by considering certain special comets. The complete work was published in 1687. A second edition was brought out in 1713 by Roger Cotes of Cambridge (1682-1716) under Newton's direc tion. The demonstrations throughout are geometrical, but are rendered unnecessarily difficult by their conciseness, and by the absence of any clue to the method by which they were obtained. The reason why the arguments were presented in a geometrical form appears to have been that the infinitesimal calculus was then unknown; and, had Newton used it to demonstrate results which were in themselves opposed to the prevalent philosophy of the time, the controversy as to the truth of his results would have been hampered by a dispute concerning the validity of the methods used in proving them. The publication of the Principia is one of the landmarks in the history of Mathematics. In it the phenomena of the solar system were shown to be deducible from laws which experience proved to be true on the earth, and thus it brought new worlds within the scope man's investigations. The conclusions were generally accepted by the leading thinkers of the time; but a generation or so had to pass before d their validity was universally admitted; henceforth, few doubted that the reign of law extended throughout the universe of non-organic matter. Newton further considered the question whether it was possible to explain gravitation as the result of other laws. He could not frame. a satisfactory hypothesis, and the problem is still unsolved. It should be noted that Newton's conclusions could not have been reached, had not observational Astronomy also developed. This was largely due to the excellent work done at Greenwich under Flamsteed (1646-1719), Halley (1656–1742), and Bradley (1692-1762), who successively occupied the position of Astronomer Royal. The last-named explained the aberration of light (1727), and thus obtained an independent determination of the velocity of light. The achievements of the seventeenth century in Astronomy and Mechanics were so great that they have thrown some of the other work of the time into comparative obscurity. The investigations in Physical Optics were, however, of singular interest. Here again Newton played the leading part. When, in 1669, he was appointed to a professorship at Cambridge, he at first chose Optics for the subject of his lectures and researches; and before the end of that year he had worked out the details of his discovery of the decomposition of a ray of white light into rays of different colours by means of a prism, from which the explanation of the phenomenon of the rainbow followed. In consequence of a chapter of accidents he failed to correct the chromatic aberration of two colours by means of a couple of prisms; hence he abandoned the hope of making refracting telescope which should be achromatic, and, instead, designed reflecting telescope, which is of a somewhat different design from those suggested by James Gregory and N. Cassegrain. We have already explained how Newton deduced the motions of the solar system from the one assumption of universal gravitation. The similar problem in Optics was the possibility of making a single hypothesis from which all the known optical phenomena could be deduced. Two plausible theories of this kind had been already suggested. In one, known as the "corpuscular "or "emission" theory, it is assumed that a luminous object emits corpuscles which hit or affect the eye. In the other, known as the wave or undulatory theory, it is assumed that light is caused by a series of waves in an ether which fills space, the waves being set in motion by pulsations of the luminous body. It would seem that at one time Newton deemed the latter the more probable hypothesis; but, though he could thus account for the phenomena of reflexion, refraction, and colours, it failed (as then propounded) to explain the rectilinear propagation of light; and this he considered fatal to its claims. He accordingly turned to the corpuscular theory, and from it deduced the phenomena of reflexion, refraction, colours, and diffraction. To do this, however, he was obliged to add a somewhat artificial rider, that the corpuscles had alternating fits of easy C. M. H. V. 46 722 Development of Physics reflexion and easy refraction, communicated to them by an ether which filled space. His various researches on the subject were embodied in his Optics published in 1704. The wave theory had been roughly outlined in 1665 by Robert Hooke. It was elaborated in a paper by Huygens in 1678, and expounded at greater length in his Traité de la Lumière, published in 1690. From it Huygens deduced the laws of reflexion, refraction, and double refraction. He was acquainted with the phenomena of polarisation; but he was unable to explain them since he assumed the vibrations in the ether to be longitudinal. It was not until the nineteenth century, when Fresnel worked out the theory on the hypothesis that the vibrations were transverse, that it was put on a satisfactory basis. Huygens was among the most illustrious mathematicians of his age, and the wave theory may be fairly deemed to be due to him. The immense reputation of Newton induced a general acceptance at the time of the corpuscular theory as enunciated by him- an unfortunate result of his extraordinary achievements, and the more curious because his writings show that on some grounds he deemed the wave theory the more probable. In science, as in other subjects, too much reliance should not be placed on individual authority. The theory of Hydrodynamics, including therein Sound and vibrations of fluids, may be said to have been created by Newton in the second book of his Principia. He determined experimentally the velocity of sound in air and other media. The difficulties of mathematical analysis involved are great, and he was not able to carry the theory very far. In connexion with the theory of Sound, may also be mentioned the names of Brook Taylor, who gave the theory of the transverse vibrations of strings, Joseph Sauveur (1653-1715), and Francis Hauksbee (1650-1713). As to other physical subjects, we may say that in all of them, at this time, there was intelligent observation and experiment. In particular the subject of Heat was attacked on the right lines by Boyle, Hooke, Newton and others, though the experimental data available were but slight. So, too, as to the work of the time in Electricity, which attracted the attention of Boyle, Halley, Newton, Picard, and Hauksbee. The death of Newton and the separation of the British school of mathematicians from their continental contemporaries may be taken as marking the close of an epoch. At the beginning of the seventeenth century Mathematics were only just breaking free from their medieval trammels, and Physics in the modern sense were non-existent. In but little more than a century Mathematics had been developed into an instrument of great power; the value of the calculus had been recognised, and the foundations of modern analysis laid; the theories of Mechanics and gravitation had been established; and the problems of Physical Optics had been subjected to mathematical processes. In this extraordinary extension of knowledge all the leading nations of Europe had Development of Natural Sciences. -Human Anatomy 723 taken part. Galileo, Descartes, Fermat, Huygens, Leibniz, and above all, Newton, form a group of workers which will be ever memorable in the history of science; and the fabric of modern Mathematics and Physics is but the superstructure erected on the foundations which they laid. (2) OTHER BRANCHES OF SCIENCE The seventeenth century may, in a broad way, be spoken of as the period during which the Natural Sciences - according to our modern classification of them-Botany, Zoology, Anatomy, Physiology, Geology, and, we may add, Chemistry, took definite shape, and began to be built up, each in its own way, as an independent branch of knowledge. The labours of the eighteenth and nineteenth centuries were, in their turn, largely directed towards carrying forward what had then been begun. But the impulse which led to this great development is to be found in the preceding century, or even earlier in the revolt against the scholastic spirit which formed so large a part of the Renaissance. The sciences in question, though having their birth partly in mere natural curiosity, sprang largely from the Art of Medicine. The treatment of disease led to enquiry into the structure and action of the body of man, and this in turn to the study of animals. The use of herbs as remedies moved men to observe the features and qualities of plants; and the science of Chemistry, though it began as Alchemy in the search for the transmutation of metals, and continued to be supported by the needs of industrial life, was in the main developed by the desire to find substances which should cure diseases. In the sixteenth century, and long afterwards, the men who were building up the several natural sciences were to be found among the teachers of the medical schools. Hence it is not wonderful that the first great triumph of the revolt against the scholastic spirit, though it won in a limited and strictly medical branch of knowledge, namely Human Anatomy, served as a bright example to nearly all the branches of natural knowledge, and exerted a powerful influence upon them. In Human Anatomy the scholastic spirit remained supreme up to the middle of the sixteenth century. The far-reaching, almost inspired labours of Galen had in quite early times produced a system of doctrines touching the structure and functions of the body of man so complete and consistent that it seemed to supply all that was needed to be known; the study of these things came to mean the study of Galen, the written page was the authority, and enquiry was narrowed to interpretation. In 1543 Andreas Vesalius (1514-64), a young professor at Padua, published a book on the structure of the human body, based, not on what Galen taught, but on what Vesalius had himself seen, and what anybody might 724 Bacon and the new method This was a powerful exemplar of the see who looked with adequate care. new method of appealing to nature to things as they are, instead of to authority. Others in modern times had dissected human bodies and appealed to these dissections as tests of truth-notably Mundinus, Carpi and Leonardo da Vinci- but none of them had produced a work so complete and so convincing as that of Vesalius. So convincing, indeed, was it that it may truly be said to have almost at one blow freed Human Anatomy from the old scholastic bonds and set it up as a striking model of the new system. The success of the work was largely due to the nature of the study. To prove his statements, to show that in this or that Galen was wrong, Vesalius had no need to use elaborate arguments or to appeal to carefully devised experiments; he had only to lay bare the structure with his scalpel, and to ask his pupils to use their eyes. The path opened up by Vesalius was followed by his pupils Fallopius, Fabricius and others; by the end of the century, the student of Human Anatomy in every medical school had ceased to ask what Galen had written, and only cared to know what his own eyes could teach him. The brilliant success thus gained by the new method applied to Human Anatomy could not fail to have an influence on other branches of learning, supported as that influence soon was by the striking results of the same new method in Mechanics and Physics. How completely this new method had laid hold of men's minds is shown by the brilliant exposition of it given by Francis Bacon (1560-1626). Though his published works belong to the seventeenth century, the Proficience and Advancement of Learning appearing in 1605 and the Novum Organum in 1620, Bacon's main ideas had come to him in the closing years of the preceding century. In the two works just mentioned, and in others, some published in his lifetime, and others at various times after his death, he elaborated in a formal exposition the principles of the method of investigating nature - the new method which, as we have just said, was being adopted by enquirers everywhere in all branches of natural knowledge. He went further: he drew up the outlines, and laboured to the time of his death to fill in the details, of a plan for the scientific work of the future, a programme of the steps to be taken in all branches of science in order to gather in with the least waste of time and labour, and in the most effective manner, the fruits of scientific enquiry. He made no notable contribution of his own to the advancement of natural knowledge; there is no evidence that the men who in his own time and in the times immediately following were actively and effectively engaged in advancing natural knowledge were in any special way influenced by his writings; indeed one of the greatest of these enquirers spoke slightingly of them. "He philosophises," said Harvey, “like a Lord Chancellor." And not only was no effort made by subsequent enquirers to carry out Bacon's programme, but the history of scientific discovery has shown that his forecast of how scientific work ought to be |