Huygens' theory

RAD PHYS teoría de Huygen f

SUBJECT AREA: Photography, film and optics

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Galileo began studying medicine at the University of Pisa but soon turned to his real interests, mathematics, mechanics and astronomy. He became Professor of Mathematics at Pisa at the age of 25 and three years later moved to Padua. In 1610 he transferred to Florence. While still a student he discovered the isochronous property of the pendulum, probably by timing with his pulse the swings of a hanging lamp during a religious ceremony in Pisa Cathedral. He later designed a pendulum-controlled clock, but it was not constructed until after his death, and then not successfully; the first successful pendulum clock was made by the Dutch scientist Christiaan Huygens in 1656. Around 1590 Galileo established the laws of motion of falling bodies, by timing rolling balls down inclined planes and not, as was once widely believed, by dropping different weights from the Leaning Tower of Pisa. These and other observations received definitive treatment in his Discorsi e dimostrazioni matematiche intorno a due nuove scienzi attenenti alla, meccanica (Dialogues Concerning Two New Sciences…) which was completed in 1634 and first printed in 1638. This work also included Galileo's proof that the path of a projectile was a parabola and, most importantly, the development of the concept of inertia.

In astronomy Galileo adopted the Copernican heliocentric theory of the universe while still in his twenties, but he lacked the evidence to promote it publicly. That evidence came with the invention of the telescope by the Dutch brothers Lippershey. Galileo heard of its invention in 1609 and had his own instrument constructed, with a convex object lens and concave eyepiece, a form which came to be known as the Galilean telescope. Galileo was the first to exploit the telescope successfully with a series of striking astronomical discoveries. He was also the first to publish the results of observations with the telescope, in his Sidereus nuncius (Starry Messenger) of 1610. All the discoveries told against the traditional view of the universe inherited from the ancient Greeks, and one in particular, that of the four satellites in orbit around Jupiter, supported the Copernican theory in that it showed that there could be another centre of motion in the universe besides the Earth: if Jupiter, why not the Sun? Galileo now felt confident enough to advocate the theory, but the advance of new ideas was opposed, not for the first or last time, by established opinion, personified in Galileo's time by the ecclesiastical authorities in Rome. Eventually he was forced to renounce the Copernican theory, at least in public, and turn to less contentious subjects such as the "two new sciences" of his last and most important work.

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SUBJECT AREA: Horology, Ports and shipping

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John Harrison was the son of a carpenter and was brought up to that trade. He was largely self-taught and learned mechanics from a copy of Nicholas Saunderson's lectures that had been lent to him. With the assistance of his younger brother, James, he built a series of unconventional clocks, mainly of wood. He was always concerned to reduce friction, without using oil, and this influenced the design of his "grasshopper" escapement. He also invented the "gridiron" compensation pendulum, which depended on the differential expansion of brass and steel. The excellent performance of his regulator clocks, which incorporated these devices, convinced him that they could also be used in a sea dock to compete for the longitude prize. In 1714 the Government had offered a prize of £20,000 for a method of determining longitude at sea to within half a degree after a voyage to the West Indies. In theory the longitude could be found by carrying an accurate timepiece that would indicate the time at a known longitude, but the requirements of the Act were very exacting. The timepiece would have to have a cumulative error of no more than two minutes after a voyage lasting six weeks.

In 1730 Harrison went to London with his proposal for a sea clock, supported by examples of his grasshopper escapement and his gridiron pendulum. His proposal received sufficient encouragement and financial support, from George Graham and others, to enable him to return to Barrow and construct his first sea clock, which he completed five years later. This was a large and complicated machine that was made out of brass but retained the wooden wheelwork and the grasshopper escapement of the regulator clocks. The two balances were interlinked to counteract the rolling of the vessel and were controlled by helical springs operating in tension. It was the first timepiece with a balance to have temperature compensation. The effect of temperature change on the timekeeping of a balance is more pronounced than it is for a pendulum, as two effects are involved: the change in the size of the balance; and the change in the elasticity of the balance spring. Harrison compensated for both effects by using a gridiron arrangement to alter the tension in the springs. This timekeeper performed creditably when it was tested on a voyage to Lisbon, and the Board of Longitude agreed to finance improved models. Harrison's second timekeeper dispensed with the use of wood and had the added refinement of a remontoire, but even before it was tested he had embarked on a third machine. The balance of this machine was controlled by a spiral spring whose effective length was altered by a bimetallic strip to compensate for changes in temperature. In 1753 Harrison commissioned a London watchmaker, John Jefferys, to make a watch for his own personal use, with a similar form of temperature compensation and a modified verge escapement that was intended to compensate for the lack of isochronism of the balance spring. The time-keeping of this watch was surprisingly good and Harrison proceeded to build a larger and more sophisticated version, with a remontoire. This timekeeper was completed in 1759 and its performance was so remarkable that Harrison decided to enter it for the longitude prize in place of his third machine. It was tested on two voyages to the West Indies and on both occasions it met the requirements of the Act, but the Board of Longitude withheld half the prize money until they had proof that the timekeeper could be duplicated. Copies were made by Harrison and by Larcum Kendall, but the Board still continued to prevaricate and Harrison received the full amount of the prize in 1773 only after George III had intervened on his behalf.

Although Harrison had shown that it was possible to construct a timepiece of sufficient accuracy to determine longitude at sea, his solution was too complex and costly to be produced in quantity. It had, for example, taken Larcum Kendall two years to produce his copy of Harrison's fourth timekeeper, but Harrison had overcome the psychological barrier and opened the door for others to produce chronometers in quantity at an affordable price. This was achieved before the end of the century by Arnold and Earnshaw, but they used an entirely different design that owed more to Le Roy than it did to Harrison and which only retained Harrison's maintaining power.

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See also: Burgi, Jost; Huygens, Christiaan

SUBJECT AREA: Domestic appliances and interiors

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Largely educated by his father, he worked for some time for Huygens at Ley den, then for a time in London where he assisted Robert Boyle with his experiments on the air pump. He supposedly invented the double-acting air pump. He travelled to Venice and worked there for a time, but was back in London in 1684 before taking up the position of Professor of Mathematics at the University of Marburg (in 1669 or 1670 he became a Doctor of Medicine at Angers), where he remained from 1687 to 1695. Then followed a period at Cassel, where he was employed by the Duke of Hesse. In this capacity he was much involved in the application of steam-power to pumping water for the Duke's garden fountains. Papin finally returned to London in 1707. He is best known for his "digester", none other than the domestic pressure-cooker. John Evelyn describes it in his diary (12 April 1682): "I went this Afternoone to a Supper, with severall of the R.Society, which was all dressed (both fish and flesh) in Monsieur Papins Digestorie; by which the hardest bones of Biefe itself, \& Mutton, were without water, or other liquor, \& with less than 8 ounces of Coales made as soft as Cheeze, produc'd an incredible quantity of Gravie…. This Philosophical Supper raised much mirth among us, \& exceedingly pleased all the Companie." The pressure-cooker depends on the increase in the boiling point of water with increase of pressure. To avoid the risk of the vessel exploding, Papin devised a weight-loaded lever-type safety valve.

There are those who would claim that Papin preceded Newcomen as the true inventor of the steam engine. There is no doubt that as early as 1690 Papin had the idea of an atmospheric engine, in which a piston in a cylinder is forced upwards by expanding steam and then returned by the weight of the atmosphere upon the piston, but he lacked practical engineering skill such as was necessary to put theory into practice. The story is told of his last trip from Cassel, when returning to England. It is said that he built his own steamboat, intending to make the whole journey by this means, ending with a triumphal journey up the Thames. However, boatmen on the river Weser, thinking that the steamboat threatened their livelihood, attacked it and broke it up. Papin had to travel by more orthodox means. Papin is said to have co-operated with Thomas Savery in the development of the lat-ter's steam engine, on which he was working c. 1705.

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