V. PHYSICS

Physics and chemistry conflicted less visibly than geography and biology with the ancient creed, for they dealt with solids, liquids, and gases that apparently had no connection with theology; but even in that material realm the progress of science was enlarging the rule of law, and weakening the faith in miracles. The study of physics rested on no philosophical interests, but on commercial and industrial needs.

Navigators, having induced astronomers to chart the skies more accurately, now offered rewards for a clock that would aid in finding longitude despite the perturbations of the sea. Longitude at sea could be determined by comparing the moment of sunrise or meridian with the time shown at that instant by a clock set to keep Greenwich or Paris time; but unless the clock was accurate the calculation would be dangerously wrong. In 1657 Huygens contrived a reliable clock by attaching a pendulum to a toothed escapement wheel, but such a clock was useless on a rolling and pitching ship.* After many trials, Huygens constructed a successful marine clock by substituting for the pendulum a balance wheel worked by two springs. This was among the illuminating suggestions expounded by him in one of the classics of modern science, the Horologium oscillatorium (The Pendulum Clock) published in Paris in 1673. Three years later Hooke invented the anchor escapement of clocks, applied spiral springs to the balance wheel of watches, and expounded the action of the springs on the principle Ut tensio sic vis—“As the tension, so the force”; this is still called Hooke’s law. Pocket watches could now be made more competently and cheaply than before.

In the Horologium and a special monograph Huygens studied the law of centrifugal force—that every particle of a rotating body not lying in the axis of rotation is subject to a centrifugal force which increases with its distance from the axis, and with the speed of rotation. He set a clay sphere rotating rapidly, and found that it assumed the form of a spheroid flattened at both ends of the axis. On this centrifugal principle he explained the polar flattening of the planet Jupiter, and by analogy he concluded that the earth too must be slightly flattened at the poles.

Huygens’ Tractatus de Motu Corporum ex Percussione (1703), published eight years after his death, continued the studies of Galileo, Descartes, and Wallis on the problems of impact. These presented intriguing mysteries, from the play of billiards to the collision of stars. How is force transmitted from a moving object to an object that it strikes? Huygens did not solve the mystery, but he stated some basic principles:

I. If upon a body at rest another body equal to it impinges, the latter will come to rest after the impact, while the body initially at rest will acquire the velocity of the impinging body.

II. If two equal bodies collide with unequal velocities, they will move, after impact, with interchanged velocities. . . .

XI. In the mutual impact of two bodies the sum of the products of the masses into the squares of the relative velocities is the same before and after impact.

These propositions, formulated by Huygens in 1669, gave partial expression to the most comprehensive principle of modern physics, the conservation of energy. They were, however, only ideally true, since they assumed complete elasticity in bodies. As no body in nature is perfectly elastic, the relative velocity of impinging objects is diminished according to the substance of which they are composed. Newton determined this rate of diminution for wood, cork, steel, and glass in the introductory scholium to Book I of hisPrincipia (1687).

Another line of investigation flowed from the experiments of Torricelli and Pascal on atmospheric pressure. Pascal had announced in 1647 that “any vessel, however large, can be made empty of all matter known in nature and perceptible to the senses.” 37 For hundreds of years European philosophy had proclaimed that natura abhorrat vacuum; even now a Paris professor informed Pascal that the angels themselves could not produce a vacuum, and Descartes scornfully remarked that the only existing vacuum was in Pascal’s head. But about 1650 Otto von Guericke constructed at Magdeburg an air pump which produced so nearly complete a vacuum that he astonished the dignitaries of his country, and the luminaries of the scientific world, with a famous experiment known as “the Magdeburg hemispheres” (1654). In the presence of the Emperor Ferdinand III and the Imperial Diet at Ratisbon, he brought two bronze hemispherical shells together in such a way that they were hermetically sealed but not mechanically connected at their edges; he pumped nearly all the air from their united interiors; then he showed that the combined strength of sixteen horses—eight pulling in one direction, eight in the opposite direction—could not separate the two halves of the sphere; but when a stopcock in one hemisphere was opened, admitting air, the shells could be separated by hand.

Guericke had a flair for making physics intelligible to emperors. By emptying a copper sphere of water and air he caused it to collapse with a loud and startling noise; so he demonstrated the pressure of the atmosphere. He balanced two equal globes, and made one fall by pumping air from the other; so he proved that air has weight. He confessed that all vacuums were incomplete, but he showed that in his imperfect vacuums a flame would be extinguished, animals would suffocate, and a striking clock would make no sound; so he prepared for the discovery of oxygen, and revealed air as the medium of sound. He used the suction of a vacuum to pump water and raise weights, and shared in preparing for the steam engine. Having become burgomaster of Magdeburg, he delayed publishing his discoveries till 1672; but he communicated them to Kaspar Schott, Jesuit professor of physics at Würzburg, who printed an account of them in 1657. It was this publication that stimulated Boyle to the researches leading to the law of atmospheric pressure.

Robert Boyle was a prime factor in the flowering of English science in the second half of the seventeenth century. His father, Richard Boyle, Earl of Cork, had acquired a large estate in Ireland, and Robert inherited most of this at the age of seventeen (1644). On frequent visits to London he became acquainted with Wallis, Hooke, Wren, and other members of the “Invisible College.” Fascinated by their work and their aspirations, he moved to Oxford and built a laboratory there (1654). He was a man of warm enthusiasms and of a piety that no science could destroy. He refused to communicate further (through Oldenburg) with Spinoza when he learned that the philosopher worshiped “substance” as God; but he placed much of his fortune at the service of science, and helped many friends. Tall and lean, frail and often ill, he held death at a distance by resolute diet and regimen. He found in his laboratory “that water of Lethe which causes me to forget everything but the joy of making experiments.” 38

Having read of Guericke’s air pump, Boyle devised, with Hooke’s help (1657), a “pneumatic engine” to study the properties of the atmosphere. With this and later pumps he proved that the column of mercury in a barometer is supported by atmospheric pressure, and he measured roughly the density of the air. He advanced upon Galileo’s alleged experiment at Pisa by showing that even in an incomplete vacuum a bunch of feathers fell as rapidly as a stone. He showed that light is not affected by a vacuum, and therefore does not, like sound, use air as its medium of transmission; and he confirmed Guericke’s demonstration that air is indispensable to life. (When a mouse fainted in the vacuum chamber he stopped the experiment and revived it by letting in air.) We see the international of science in action when we learn that Guericke was stimulated by Boyle’s work to contrive a better air pump and resume his scientific studies; and that Huygens, visiting Boyle in 1661, was led to make similar instruments and tests.

Boyle went on to creative inquiries into refraction, crystals, specific gravities, hydrostatics, and heat. He crowned his contributions to physics by formulating the law that bears his name: that the pressure of air or any gas varies inversely as its volume—or that at a constant temperature the pressure of a gas, multiplied by its volume, is constant. He first announced this principle in 1662, and generously credited it to his pupil Richard Towneley. Hooke had reached the same formula in 1660 by independent experiments, but did not make it known till 1665. A French priest, Edme Mariotte, about the same time as Boyle, arrived at a similar conclusion—“air is compressed according to the weight acting upon it”; he published this in 1676; and on the Continent his name, rather than Boyle’s, is attached to the law of atmospheric pressure. Whatever its parentage, it was one of the progenitors of the steam engine and the Industrial Revolution.

Boyle and Hooke followed up Bacon’s view that “heat is a motion of expansion not uniformly of the whole body, but in the smaller parts of it.” 39 Describing heat as “a property arising in a body from the motion or agitation of its parts,” Hooke distinguished it from fire and flame, which he attributed to the action of air on heated bodies. “All bodies,” said Hooke, “have some degree of heat in them,” for “the parts of all bodies, though never so solid, do yet vibrate”; 40 cold is merely a negative conception. Mariotte amused his friends by showing that “cold” could burn: with a concave slab of ice he focused sunlight upon gunpowder, causing it to explode. Spinoza’s friend Count Ehrenfried Walter von Tschirnhaus melted porcelain and silver dollars by focusing upon them the light of the sun.

In the physics of sound two Englishmen, William Noble and Thomas Pigot, separately showed (c. 1673) that not only the whole, but different parts of a string may vibrate with diverse overtones, in sympathy with a near and related string plucked, struck, or bowed. Descartes had suggested this to Mersenne, and Joseph Sauveur, working on this idea, arrived independently at results similar to those of the Englishmen (1700); we should note in passing that Sauveur, who first used the word acoustics, had been a deaf mute from infancy. 41 In 1711 John Shore invented the tuning fork. Attempts to find the velocity of sound were made in this period by Borelli, Viviani, Picard, Cassini, Huygens, Flamsteed, Boyle, Halley, and Newton; Boyle, reckoning it at 1,126 feet per second, came closest to our current estimate. William Derham pointed out (1708) that this knowledge could be used to calculate the distance of a storm by observing the time interval between the lightning flash and the thunderclap.

The second half of the seventeenth century was probably the most brilliant epoch in the history of the physics of light. First, what was light itself? Hooke, always ready to delve into difficulties, hazarded the view that light is “nothing else but a peculiar motion of the parts of the luminous body” 42—i.e., light differs from heat only in the more rapid motion of the body’s constituent particles.* Second, how fast does it move? Scientists had heretofore assumed that the speed of light was infinite, and even the venturesome Hooke had regarded it as in any case too great for measurement. In 1675 Olaus Roemer, a Danish astronomer brought to Paris by Picard, proved the finite velocity of light by noting that the period of eclipse of Jupiter’s innermost satellite varied according as the earth was moving toward or away from that planet; by computations based on the time of the satellite’s revolution and the diameter of the earth’s orbit, he showed that the variation in the observed time of the eclipse was due to the time taken by light from the satellite to traverse the orbit of the earth; and on that slender basis he calculated the speed of light at some 120,000 miles per second. (The current estimate is 186,000 miles.)

But how was light transmitted? Did it move in straight lines? If so, how did it get around corners? Francesco Grimaldi, Jesuit professor at Bologna, discovered and named (1665) the phenomena of diffraction—that rays of light passing through a small opening into a dark room spread more widely on the opposite wall than straight lines from source to wall would warrant, and that rays of light are slightly deflected from a straight line when they pass by the edges of an opaque body; these and other findings led Grimaldi to accept Leonardo da Vinci’s suggestion that light moves in expanding waves. Hooke agreed, but it was Huygens who established the wave theory still popular among physicists. In another classic of modern science, Traité de la lumière (1690), Huygens reported the conclusions he had reached from studies begun twelve years before: that light is transmitted by a hypothetical substance which he called aether (from the Greek word for the sky), and which he conceived as made up of small, hard, elastic bodies transmitting light in successive spherical waves spreading out from the luminous source. On this theory he formulated laws of reflection, refraction, and double refraction; he ascribed to the enveloping motion of the waves the ability of light to move around corners and opaque objects; and he explained translucence by supposing the ether particles to be so minute that they can travel around and between the particles composing transparent liquids and solids. But he confessed himself unable to explain polarization; this was one of the reasons why Newton rejected the wave hypothesis and preferred the corpuscular theory of light.

The seventeenth century made only modest advances in the study of electricity after the work of Gilbert and Kircher on magnetism, and of Cabeo on electrical repulsion. Halley studied the influence of terrestrial magnetism on compass needles, and was the first to recognize a connection between the magnetism of the earth and the aurora borealis (1692). Guericke reported in 1672 some experiments in frictional electricity. A ball of sulphur, after being rotated against his hand, attracted paper, feathers, and other light objects, and carried them around with it in its rotation; he likened this to the action of the earth carrying along with it the objects on or near its surface. He verified electrical repulsion by showing that a feather, placed between the electrified ball and the floor, jumped up and down from one to the other. He pioneered in studying conduction by proving that an electric charge could travel along a linen thread, and that bodies could become electrified by being brought near to the electrified ball. Francis Hauksbee, of the Royal Society, developed (1705–9) a better method of generating electricity by rapidly rotating an exhausted glass ball, and then applying it to his hand; the contacts gave off sparks an inch long, providing light enough for reading. Another Englishman, Wall, having produced similar sparks, likened their sound and light to thunder and lightning (1708). Newton made the same comparison in 1716; Franklin confirmed the relation in 1749. So, year by year, and mind by mind, the impenetrable immensity surrenders some teasing, luring fragment of its mystery.

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