Science

The Renaissance.

We now reach the period in the history of the world known as the Renaissance, when increased material prosperity and improved political stability in the 15th and 16th centuries gave many converging streams of thought opportunity to unite. The Renaissance was not, as it is sometimes represented, a sudden break with mediaevalism and a birth of the modern world. A number of conditions favourable to rapid development happened to coincide, and, in the course of a, century, men's out look on themselves and on nature became profoundly modified. The recovery of the Greek language, the voyages of Columbus, the decay of the Western and the passing of the Eastern empire, the temporary diminution in power of the papacy, the invention of printing, all tended to produce new ideas and to prepare men's minds to accept the more human and naturalistic view of the universe which had been current among the Greeks, in place of the theologicalaspect which it wore to the mediaeval schoolmen and ecclesiastics. At first the tendency was to substitute the authority of the ancients for the authority of the schoolmen, but gradually more independence of thought was secured ; men like Leonardo da Vinci (1452-1519) began to observe and to experiment ; Nicolaus Copernicus (1473-1543) revived the heliocentric theory, and showed how the accumulated mass of astronomical observa tions could be interpreted by its means ; and Vesalius (1514 1564) took up again the study of anatomy, which gradually made its way in the schools of medicine, in face of the prejudice against mutilating the human body.

Physics.

In physics the true method was first used freely by Galileo Galilei (1564-1642), who, turning the newly invented tele scope to examine the stars and planets, increased enormously the evidence for the theory of Copernicus. But a greater achievement was the foundation of the science of dynamics. We have seen how the Aristotelians held the belief that every body sought its natural place, the place of heavy bodies being below and that of light ones above. Innate qualities of heaviness and lightness were thus in voked, and it was believed that heavy things always fell faster than light ones. Galileo, rightly rejecting the current point of view, set himself to examine not why, but how, things fell. First he showed that, save for a small error due to the resistance of the air, heavy and light bodies fall at the same rate. Next, by trying the consequences of one hypothesis after another till he found one that was both self-consistent and gave deductions in accord ance with experiment, he proved that the space traversed by a falling body is proportional to the square of the time of fall. To verify this result experimentally, Galileo convinced himself that a body falling down an inclined plane acquired a speed which is the same as that it would have attained in falling through the same vertical height. He could therefore use a slow movement down a

plane for his experiments instead of the unmanageably rapid course of a body falling freely. Nor was this all. From this stage of the investigation another consequence of his results was found to spring. A ball after running down an inclined plane of a cer tain height will run up another plane of the same height irre spective of its inclination—that is, if friction be small. The second plane may be made very long, but still, if its final height be not greater than that of the first, the ball will reach its end. Hence it is the height that matters; none of the speed of the ball is de stroyed unless it rises. If the second plane be made horizontal, the ball will thus run on for ever unless stopped by friction or some other applied force. This fundamental result, put into definite words by Newton, is known as the first law of motion, and is the foundation of the whole science of dynamics. In Galileo's day it was an entirely new conception. It had been assumed that every motion required some cause or force to maintain it. Hence arose the need of hypothetical vortices to maintain planetary move ments, and similar complications in other branches of mechanics. But it now became evident that it was not the continuous motion of the planets which needed explanation, but the constant deflection of that motion from the straight path it would hold if no applied force were in action. The way was open for Newton.

Sir Isaac Newton (1642-1727) proved mathematically that the observed motion of the planets about the sun could be explained oy the supposition that the sun exerted a force on each planet proportional inversely to the square of its distance from the planet. But the earth, at any rate, does seem to attract bodies on or near its surface, the phenomenon being the familiar but myste rious gravity. Is this force competent to account for the motion of the moon round the earth? On the assumption of the law of inverse squares, Newton calculated what the force of gravity would be at the distance of the moon and got a fair concordance with facts. But he put aside his calculations till, some years later, he proved mathematically that a sphere would attract other bodies as though all its mass were collected at its centre. He could then treat the problem accurately and thus found that the fall of a stone to the earth and the sweep of the moon in her orbit were due to the same cause. The mechanism by means of which the force is exerted remained unrevealed to Newton, and baffled all inquirers till Einstein showed that this problem was soluble by quite other means. But Newton's discovery that all the move ments of the heavens could be described by one simple law was the first great physical synthesis, and perhaps represents the highest achievement in the history of science.