A HISTORY OF PHYSICS THE GREEKS IN mathematics, metaphysics, literature, and art the Greeks displayed wonderful creative genius, but in natural science they achieved comparatively little. It would not be correct to say that they possessed little or no aptitude for observing natural phenomena, but it is true that, as a rule, they were ignorant of the art of experimentation, and that many of their physical speculations were vague, trifling, and worthless. As compared with the vast amount of theoretical deduction about nature, the number of experiments known to have been performed by the Greeks is surprisingly small. Little or no attempt was made to verify speculation by experimental evidence. As a conspicuous example of misty philosophizing we give Aristotle's proof that the world is perfect: "The bodies of which the world is composed are solids, and therefore have three dimensions. Now, three is the most perfect number, —it is the first of numbers, for of one we do not speak as a number, of two we say both, but three is the first number of which we say all. Moreover, it has a beginning, a middle, and an end." 1 MECHANICS Mechanical subjects are treated in the writings of Aristotle. The great peripatetic had grasped the notion of the parallelo 1 De Cœlo, I. 1, as translated by Whewell. gram of forces for the special case of the rectangle. He attempted the theory of the lever, stating that a force at a greater distance from the fulcrum moves a weight more easily because it describes a greater circle. He resolved the motion of a weight at the end of the lever into tangential and normal components. The tangential motion he calls according to nature; the normal motion contrary to nature. The modern reader will readily see that the expression contrary to nature applied to a natural phenomenon is inappropriate and confusing. Aristotle's views of falling bodies are very far from the truth. Nevertheless they demand our attention, for the reason that, during the Middle Ages and Renaissance, his authority was so great that they play an important rôle in scientific thought. He says: "That body is heavier than another which, in an equal bulk, moves downward quicker." In another place he teaches that bodies fall quicker in exact proportion to their weight. No statement could be further from the truth. 2 A modern writer endeavours to exonerate Aristotle as a physicist. "If he could have had any modern instrument of observation such as the telescope or microscope, or even the thermometer or barometer-placed in his hands, how swiftly would he have used such an advantage!". 3 But in the case of falling bodies, the experiment was within his reach. If it had only occurred to him, while walking up and down the paths near 1 De Cœlo, IV. 1, p. 308. 2 This law is assumed by him in the following reasoning: a without weight, but ẞ possessing weight; and let a pass over a space yo, but ẞ in the same time pass over a space ye, for that which has weight will be carried through the larger space. If now the heavy body be divided in the proportion that space ye bears to yd, and if the whole is carried through the whole space ye, then it must be that a part in the same time would be carried through yd. .." - De Cœlo, Book III., Ch. II. ... Article "Aristotle " in Encyclopædia Britannica, Ninth Edition. his school in Athens, to pick up two stones of unequal weight and drop them together, he could easily have seen that the one of, say, ten times the weight did not descend ten times faster. Immeasurably superior to Aristotle as a student of mechanics is Archimedes (287(?)-212 B.C.). He is the true originator of mechanics as a science. To him we owe the theory of the centre of gravity (centroid) and of the lever. In his Equiponderance of Planes he starts with the axiom that equal weights acting at equal distances on opposite sides of a pivot are in equilibrium, and then endeavours to establish the principle that "in the lever unequal weights are in equilibrium only when they are inversely proportional to the arms from which they are suspended." His appreciation of its efficiency is echoed in the exclamation attributed to him: "Give me a fulcrum on which to rest, and I will move the earth." We reproduce from a mechanical work of Varignon, published in Paris in 1687, a figure (Fig. 1) illustrating this saying. The Latin motto in the figure may be rendered thus: "Touch it and you will move it." 1 Consult The Works of Archimedes, edited in modern notation, with introductory chapters, by T. L. HEATH. Cambridge, University Press. While the Equiponderance treats of solids or the equilibrium of solids, the book on Floating Bodies treats of hydrostatics. His attention was first drawn to the subject of specific gravity when King Hieron asked him to test whether a crown, professed by the maker to be pure gold, was not alloyed with silver. The story goes that our philosopher was in a bath when the true method of solution flashed on his mind. He immediately leaped from the bath and ran home, shouting, "I have found it!" To solve the problem he took a piece of gold and a piece of silver, each weighing the same as the crown. According to one author, he determined the volume of water displaced by the gold, silver, and crown respectively, and calculated from that the amount of gold and silver in the crown. According to another writer, he weighed separately the gold, silver, and crown, while immersed in water, thereby determining their loss of weight in water. From these data he easily found the solution. It is possible that Archimedes solved the problem by both methods. 2 In his Floating Bodies Archimedes established the important principle, known by his name, that the loss of weight of a body submerged in water is equal to the weight of the water displaced, and that a floating body displaces its own weight of water. Since the days of Archimedes able minds have drawn erroneous conclusions on liquid pressure. The expression "hydrostatic paradox" indicates the slippery nature of the subject. All the more must we admire the clearness of conception and almost perfect logical rigour which characterize the investigations of Archimedes.3 1 VITRUVIUS, IX. 3. 2 Scriptores metrologici Romani (ed. HULTSсн, pp. 124-208). 3 A valuable paper with numerous extracts from authors is CH. THUROT'S Recherches Historiques sur le Principe d'Archimède, Paris, 1869 (extrait de la Revue Archéologique, Années 1868-1869). Archimedes is said to have shown wonderful inventive genius in various mechanical inventions. It is reported that he astonished the court of Hieron by moving heavy ships by aid of a collection of pulleys. To him is ascribed the invention of war engines, and the endless screw ("screw of Archimedes") which was used to drain the holds of ships. About a century after Archimedes, there flourished Ctesibius and his pupil Heron, both of Alexandria. They contributed little to the advancement of theoretical investigation, but they displayed wonderful mechanical ingenuity. The force-pump is probably the invention of Ctesibius. The suction pump is older and was known in the time of Aristotle. According to Vitruvius, Ctesibius designed the ancient fire-engine, consisting of the combination of two force-pumps, spraying alternately. The machine had no air-chamber, and therefore could not produce a steady stream. Heron describes the fire-engine in his Pneumatica. During the Middle Ages the fire-engine was unknown. It is said to have been first used in Augsburg in 1518.1 Ctesibius is credited with the invention of the hydraulic organ, the water-clock, and the catapult. Heron showed the earliest application of steam as a motive power, in his toy, called the "eolipile" (Fig. 2). It consisted of a hollow sphere with two arms at right angles to its axis and bent in opposite directions at its ends. When steam was generated in the sphere, it escaped through the arms and caused the sphere to rotate. It was the forerunner of Barker's water-mill and the FIG. 2. 1 A. DE ROCHAS in La Nature, Vol. XI., pp. 13, 14; 1883. |