Marine Chronometer

1. INTRODUCTION 1.1 Background The 17th century was the age of travel and colonization. Many long sea voyages were undertaken to different parts of the world. This period was known as the ‘Age of Discovery’. The ‘Age of Discovery’ or rather the ‘Age of Exploration’ began around the 15h century and progressed all the way to the 18th century. This was the age where many explorers from the British Empire and European Countries set sail to discover other parts of the world for trade. Most of these voyages spanned many months or years and were voyages over seas. Hence good navigation was necessary. When the early navigators started venturing away from the sight of land they started developing methods of navigation involving the heavenly bodies. To determine a position on the earth's surface, it is necessary and sufficient to know both the latitude, longitude. Methods of determining the latitude existed during the 17th century however there were no good methods of determining the longitude. Determining latitude was relatively easy in that it could be found from the altitude of the sun at noon with the aid of a table giving the sun's declination for the day. For longitude, early ocean navigators had to rely on dead reckoning or Galileo's method Dead reckoning (DR) is the process of estimating one's current position based upon a previously determined position, or fix and advancing that position based upon known or estimated speeds over elapsed time, and course. The disadvantage of dead reckoning was that since new positions were calculated solely from previous positions, the errors of the process were cumulative, so the error in the position fix grew with time. Hence dead reckoning was inaccurate on long voyages out of sight of land and these voyages sometimes ended in tragedy as a result. Galileo's method based on observing Jupiter's natural satellites, was usually not possible at sea due to the ship's constant motion. In order to avoid problems with not knowing one's position accurately, navigators used to, where possible, take advantage of their knowledge of latitude. They used to sail to the latitude of their destination, turn toward their destination and follow a line of constant latitude. This was known as running down ‘a westing’ (if westbound, easting otherwise). This prevented a ship from taking the most direct route (a great circle) or a route with the most favorable winds and currents, extending the voyage by days or even weeks. This increased the likelihood of short rations, scurvy or starvation leading to poor health or even death for members of the crew. Hence overall this method was a risk to the ship. Errors in navigation had also resulted in shipwrecks. Most of those disasters were attributed to serious errors in reckoning position at sea. One such example was the loss of Admiral Cloudesley Shovell and his fleet. In 1707 Sir Cloudesley Shovell was returning from Gibraltar with his fleet when he sailed into cloudy weather. After 12 days with no sight of land they calculated they were west of the southern tip of England and decided to hold station. That night hey ran ground on the Scilly Isles losing 4 ships and nearly 2000 crew members including the Admiral. Hence an accurate method of determining the longitude had to be discovered. The Longitude Board was introduced. The Board of Longitude was the popular name for the Commissioners for the Discovery of the Longitude at Sea. It was a British

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Government body formed in 1714 to solve the problem of finding longitude at sea. The Board gathered the greatest scientific minds of its day to work on the problem, including Sir Isaac Newton, and also put up prizes for those who could demonstrate a working device or method. The main longitude prizes were: £10,000 for a method that could determine longitude within 60 nautical miles (111 km) £15,000 for a method that could determine longitude within 40 nautical miles (74 km) £20,000 for a method that could determine longitude within 30 nautical miles (56 km). In addition, the Board had the discretion to make awards to persons who were making significant contributions to the effort or to provide ongoing financial support to those who were working productively towards the solution. The Board could also make advances of up to £2,000 for experimental work deemed promising. It was during this time that John Harrison invented the marine chronometer. This was an instrument for determining the longitude of the place. Chronometer is a clock that measures time with great accuracy and is used in navigation to help mariner (with the help of some astronomic tables) for determining its precise location on the water while they were in the middle of the ocean. Hence it was thanks to John Harrison’s discovery that safer voyages over seas could be made. A longitude describes the location of a place on Earth east or west of a north-south line called the Prime Meridian. Longitude is given as an angular measurement ranging from 0° at the Prime Meridian to +180° eastward and −180° westward. Many solutions were proposed for how to determine longitude at the end of an exploratory sea voyage, and hence, the longitude of the place that was visited (in case, for instance, one would want to revisit the location or place it on a map). The practical methods relied on a comparison of local time with the time at a given place (such as Greenwich or Paris). Harrison instead set out to solve the problem in probably the most direct way: by producing a reliable clock. The theory was simple and had been first proposed by Frisius. The difficulty, however, was in producing a clock which could maintain accurate time on a lengthy, rough sea voyage with widely-varying conditions of temperature, pressure and humidity. Frisius had realized that to determine longitude, a clock would have to be “of great exactness”. Many leading scientists including Newton and Huygens doubted that such a clock could ever be built and had more optimism for astronomical observations (such as the Method of Lunar Distances). Huygens ran trials using both a pendulum and a spiral balance spring clock as methods of determining longitude. Although both types showed some favorable results, they were both prone to fickleness. Newton observed that “A good watch may serve to keep a reckoning at sea for some days and to know the time of a celestial observation; and for this end a good Jewel may suffice till a better sought of watch can be found out. But when longitude at sea is lost, it cannot be found again by any watch.” However, if such a clock were built and set at noon in London at the start of a voyage, it would subsequently always tell you how far from noon it was in London at that second, regardless of where you had traveled. By referring to the clock when it is

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noon locally (i.e. the Sun is at its highest in the sky where you are) you can read, almost directly from the clock face, how far around the world you are from London. For instance, if the clock shows that it is midnight in London when it is noon locally, then you are half way round the world, (e.g. 180 degrees of longitude) from London. After steadfastly pursuing various methods during thirty years of experimentation, Harrison finally designed and built the world's first successful marine chronometers, the highly accurate maritime time-keeping instruments that, for the first time, allowed a navigator to accurately assess his ship's position in longitude. This is so because the earth is constantly rotating, and therefore knowing the time whilst making an altitude measurement to a known heavenly body such as the sun, provided critical data for a ship's position east-west—a necessary capability for re-approaching land after voyages over medium and long distances. Knowing such measurements without an accurate time could only show position in latitude which was a trivial problem in comparison. Such a maritime clock had to be not only highly accurate over long time intervals, but relatively impervious to corrosion in salt air, able to tolerate wide variations in temperature and humidity and in general durable whilst able to function at the odd angles and pitch and yaw typical of decks under strong waves and storm tossed conditions. Yet the timekeeping device with such accuracy would eventually also allow the determination of longitude accurately, making the device a fundamental key to the modern age. Nonetheless, for many years even after the American Revolution, chronometers were expensive rarities, as their adoption and use proceeded slowly due to the precision manufacturing necessary and hence high expense, but by the early 19th century, navigation at sea without one was considered unwise to unthinkable. Using a chronometer to aid navigation simply saved lives and ships—the insurance industry, exercise of self-interest, and common sense did the rest in making the device a universal tool of maritime trade.

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1.2 Rationale There were other methods of navigation before the 17th century. These methods were used to determine the latitude, longitude and sometimes even the altitude. Some of these methods were accurate on land however they weren’t accurate on sea.

1.2.1 The Cross-Staff (Fore-Staff) This was invented by the Portuguese around the 13th century. Astronomers aided Prince Henry the Navigator in developing the cross-staff. The instrument was cheap enough and simple enough for use at sea. The original ones consisted of a staff 30 - 36 inches long with a shorter moveable cross piece. The staff was held to the cheekbone and lined up with the horizon. The cross-piece was moved up and down the staff until the end of it was lined up with the star. The altitude was read off the staff. In the 16th century additional transversals (cross-pieces) were added along with extra scales on the staff. This gave the navigator the ability to get a larger range of angles. In the 17th century a “brass sight” was added. This was a sight vane enabling the observer to use the back staff with his back to the sun. In the 18th century a piece of smoked glass was added to “temper” the sunlight when facing the sun. This instrument was used well into the 18th century as it was better for taking star sights than the back-staff.

Fig.1 Fore-Staff

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1.2.2 The Astrolabe The astrolabe which means “star-taker,” was in use from around 1480. It was at this time the Portuguese drew up the rules for taking an altitude of the sun to establish latitude. The mariner's astrolabe was much simpler and cheaper than the astronomer’s version. Most were made of metal but some larger ones were made of wood for use on shore. It was probably invented by the Greeks. Astrolabes were used for surveying as well as astronomy. The scale read down from the zenith, 0° so the zenith distance was read directly. There were many other scales related to astronomical work on these expensive instruments. Arabs crossing the deserts probably used one type of them long before they were used at sea. The instruments were hard to use in a heavy sea or in much wind. Later the English navigators preferred to use them for high altitudes (over 50°) and the cross-staff for lower latitudes. They had to be small (6 or 7 inches) and heavy (around 4 lbs.) in order to reduce the effect of the wind. The scales running from 0° to 90° scales were mirrored and used 0° for the horizon. They were cut by hand and often not too accurate. It could only be read to 1/2 a degree. Errors of 5° were not uncommon.

Fig. 2 The Astrolabe

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1.2.3 The Back-Staff (Davis’ Quadrant) Around 1594 John Davis invented the back-staff for taking altitudes of the sun which was difficult to do with a cross-staff. The observer turned his back to the sun so it did not burn his eyes. The sun image was projected on to a horizon slit. The normal practice was to use the back-staff for sun sights and the cross-staff for star sights. It could measure angles up to 90 degrees. The scales were easier to graduate than the cross-staff hence more accurate. However it was more fragile and required more craftsmanship to build. The instrument had two scales and three vanes (some had four) on what looks like an A frame. The front vane mounted on the 60 degree arc had a pinhole that cast a shadow on the horizon vane. Shadow vanes were also used. The third vane on the 30 degree arc was lined up so as to get the suns image and the horizon. The front vane, on the 60 degree arc, was set to the expected altitude. This could be set roughly by looking through the instrument. The vane on the closer 30 degree arch was adjusted until the sun shadow was lined up with the horizon. The sum of the two readings gave the altitude.

Fig. 3 Davis Quardant

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1.2.4 Compass The compass allowed seafaring men to depart from the confines of the coast. Before the compass the vessel master had to rely on the stars to give him some idea of direction. The instrument seamen most depend on, whether deep sea or coastal, is the compass. The first ones were very crude. The exact dates of the development of the compass are shrouded in the fog of history. They were an iron needle floated on a basin of water. The needle had to be magnetized with a lodestone. While this procedure seems to have been used in the 15th century, some needles were mounted on a pivot as early as 1200. The needle still had to be stroked or “touched” by the lodestone. Sometime in the 13th century the needle was mounted under a card with a painting of a wind rose on it. The compass was mounted in a box and placed on a shelf. The needle could be refreshed with the lodestone from underneath. In medieval times some of the compasses were 4 or 5 inches in diameter. There were often two compasses in wood or brass boxes with a candle placed between them. The housing was called a bittacle. This was the forerunner of the modern binnacle. It seems that around 1500 the compass was mounted in gimbals. The magnetized steel bars mounted under the card arrived in the 18th century. Compasses were still fairly crude until that time.

Fig. 4 Compass

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1.2.5 Galileo's proposal — Jovian moons In 1612, having determined the orbital periods of Jupiter's four brightest satellites (Io, Europa, Ganymede and Callisto), Galileo proposed that with sufficiently accurate knowledge of their orbits one could use their positions as a universal clock and this would make possible the determination of longitude. He worked on this problem from time to time during the remainder of his life. To be successful, it required the observation of the moons from the deck of a moving ship. To this end, Galileo proposed the celatone, a device in the form of a helmet with a telescope mounted so as to accommodate the motion of the observer on the ship. This was later replaced with the idea of a pair of nested hemispheric shells separated by a bath of oil. This would provide a platform that would allow the observer to remain stationary as the ship rolled beneath him, in the manner of a gimballed platform. To provide for the determination of time from the observed moons' positions, a Jovilabe was offered — this was an analogue computer that calculated time from the positions and gets its name by its similarities to an astrolabe. The practical problems were severe and the method was never used at sea. However, it was used for longitude determination on land.

Fig. 5 Jovilabe

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Halley's proposals — lunar occultations and appulses, magnetic deviation Around 1683, Edmund Halley proposed using a telescope to observe the time of occultations or appulses of a star by the moon as a means of determining time while at sea. He had accumulated observations of the moon's position and of certain stars to this end and had deduced the means of correcting errors in predictions of the moon's position. Following John Flamsteed's death, as new Astronomer Royal, he had undertaken the task of observing both stellar positions and the path of the moon, with the intention of supplementing existing knowledge and advancing his proposal for determining longitude at sea. By this time, he had abandoned the use of occultations in preference for appulses exclusively. No reason was given by Halley for abandoning occultation’s, however, there are few bright stars occulted by the moon and the task of documenting the dim stars' positions and training navigators to recognize them would be daunting. Appulses with brighter stars would be more practical. While he had tested the method at sea, it was never widely used or considered as a viable method. His observations did contribute to the lunar distance method. Halley also hoped that careful observations of magnetic deviations could provide a determination of longitude. The magnetic field of the Earth was not well understood at the time. Mariners had observed that magnetic north deviated from geographic north in many locations. Halley and others hoped that the pattern of deviation, if consistent, could be used to determine longitude. If the measured deviation matched that recorded on a chart, the position would be known. Halley used his voyages on the pink Paramour to study the magnetic variance and was able to provide maps showing the halleyan or isogonic lines. This method was eventually to fail as the localized variations from general magnetic trends make the method unreliable.

All of the above methods of calculations couldn’t be used on sea voyages to determine the longitude of a place.

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1.3 Objectives of the Project 1.3.1

Objective • • • •

1.3.2

To provide a clear understanding of what a chronometer is. To let the reader get a glimpse into the life of John Harrison and the various hardships he faced. To give the reader the different types of chronometers (invented by Harrison) and their physical description. To explain ‘how to use a chronometer’

Methods of Reaching the Objective • • •

By providing information from various sources By providing the information in a story format By providing pictures wherever necessary

1.4 Scope of the Project 1.4.2 Scope of Work The scope of work was to get a clear understanding of what a chronometer is essentially, what the different types of chronometers are and how it can be used.

1.4.3 Limitations Finding information about the mechanism of a chronometer was difficult. Also getting proper diagrams of the mechanism of a chronometer was difficult.

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2. Product Description 2.1 Physical Description Chronometer is a clock that measures time with great accuracy and is used in navigation to help mariners (with the help of some astronomic tables) for determining their precise location on the water while they were in the middle of the ocean. Chronometers are set to Greenwich Mean Time (the basic time used in celestial navigation). The chronometer's face is the same as that of an ordinary clock or watch. It has an hour and a minute hand. However, it has two additional superimposed dials one with a second hand that moves every half second, and the other with a hand that indicates numbers of hours since the last winding. The Marine Chronometer is mounted in gimbals to keep it in a horizontal position to reduce the effect of the ship motion on the chronometer. It is fitted in a special wood box with a glass top that is usually airtight. This box is placed in a second clothpadded box to protect the instrument from vibrations and temperature changes. The chronometers that were finally adopted for use by mariners resembled large pocket watches. However a chronometer differs from an ordinary clock or watch principally in that it is considerably more accurate because it contains a variable lever device and a temperature-compensating balance. Variable lever regulates the power transmitted by the mainspring so that it remains uniform as it unwinds. Balance formed by a combination of metals of different coefficients of expansion, compensates for changes in temperature and makes the rate of losing or gaining approximately uniform at all temperatures. As mentioned above the first chronometer was invented by John Harrison. John Harrison invented 4 chronometers through his life. He spent nearly 35 years on building and modifying his inventions.

2.1.1 Harrison Number One Constructed between 1730 and 1735, H1 is essentially a portable version of Harrison's precision wooden clocks. It is spring-driven and only runs for one day (the wooden clocks run for eight days). The moving parts are controlled and counterbalanced by springs so that, unlike a pendulum clock, H1 is independent of the direction of gravity. The major improvement was that the pendulum originally used in the clock was replaced by a balance spring with two 5 pound weights connected by brass arcs. When the clock was tilted or turned by the movement of the sea, the weights attached will balance the spring and any particular movement communicated to one balance will be automatically counteracted by an equal and opposite movement of its

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opposing counterpart. The chronometer weighed 72 pounds. When the chronometer was put to a sea trial, it was relatively successful, losing only 3 seconds in 24 hours.

Fig. 6 Harrison No. 1 H1 was brought to London in 1735 and displayed to the scientific community. Harrison was besieged by requests from both scientists and socialites to see the timekeeper. In 1736, Harrison and his timekeeper travelled to Lisbon aboard the ship Centurion to test the clock, and returned on the Orford. H1 performed well in the trial, keeping time accurately enough for Harrison to correct a misreading of the Orford's longitude on the return voyage. However, Harrison did not ask for a second trial but, instead, requested financial assistance from the Board of Longitude to make a second marine timekeeper.

2.1.2 Harrison Number Two In 1739 H2 was completed. H2 was tall and heavier but it took up less deck space. The main innovation in the mechanism of H2, one which Harrison used in all his subsequent longitude time-keepers was a remontoire. The remontoire mechanism ensures that the force on the escapement is constant, thus improving the accuracy of the clock. Larger and heavier than H1, H2 is of fundamentally the same design as H1. In 1740 he realized its design was wrong. The bar balances did not always counter the motion of a ship, a deficiency that could be corrected if the balances were circular.

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Fig. 7 Harrison No. 2 Harrison requested more money from the Board to work on a third timekeeper.

2.1.3 Harrison Number Three By 1741 John Harrison had commenced H3. His aim was to achieve a uniform running of the clock. H3 was fairly similar to H2 but it was slightly shorter, lighter and had circular balances instead of dumb-bell shapes. A bi-metallic curb was used to allow for variations in temperature. However, H3 had the serious drawback of being impossible to adjust without dismantling and re-assembling, which were long procedures. Harrison worked on his third timekeeper from 1740 to 1759. After 19 years of labour, it failed to reach the accuracy required by the Board of Longitude. H3 incorporated two inventions of Harrison's – -

A bimetallic strip, to compensate the balance spring for the effects of changes in temperature A caged roller bearing, the ultimate version of his anti-friction devices.

Both of these inventions are used in a variety of machines nowadays. Despite these innovations, work on H3 seemed to lead nowhere and its ultimate role was to convince Harrison that the solution to the longitude problem lay in an entirely different design.

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Fig. 8 Harrison Number 3

2.1.4 Harrison Number Four In 1753, Harrison commissioned London watchmaker John Jefferys to make him a watch following Harrison's own designs. The watch was intended for Harrison's own personal use - to help with his astronomical observing and clock testing. No one in the 1750s thought of the pocket watch as a serious timekeeper. However, Harrison discovered with his new watch that if certain improvements were made, it had the potential to be an excellent timekeeper. In 1755, as well as asking for continued support for the construction of H3, he asked the Board of Longitude for support ... to make two watches, one of such size as may be worn in the pocket & the other bigger... having good reason to think from the performance of one already executed... that such small machines may be rendered capable of being of great service with respect to the Longitude at Sea... H4 is completely different from the other three timekeepers. Just 13 cm in diameter and weighing 1.45 kg, it looks like a very large pocket watch. Harrison's son William set sail for the West Indies, with H4, aboard the ship Deptford on 18 November 1761. They arrived in Jamaica on 19 January 1762, where the watch was found to be only 5.1 seconds slow! It was a remarkable achievement but it would be some time before the Board of Longitude was sufficiently satisfied to award Harrison the prize. A second trial of H4 was arranged and William departed for Barbados aboard the Tartar on 28 March 1764. As with the first trial, William used H4 to predict the ship's arrival at Madeira with extraordinary accuracy. The watch's error was computed to be 39.2 seconds over a voyage of 47 days, three times better than required to win the

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£20,000 longitude prize. The Board of Longitude, however, implied that the watch was a fluke and would not be satisfied unless others of the same kind could be made and tested. Harrison would be paid £10,000 as soon as he disclosed his secrets and handed over his mechanisms to the Astronomer Royal, with the remaining £10,000 being paid when other timekeepers of the same type, accurate enough to find longitude to within 30 miles, were made.

Fig. 9 Harrison Number 4 The Board of Longitude remained unconvinced. They stated that half of the prize money would be paid once Harrison had disclosed the workings of H4 to a speciallyappointed committee. They also implied that H4's accuracy was a fluke and that copies of the watch should be made and tested. Finally, all four of Harrison's timekeepers should be handed over to the Board once he had received the £10,000. Harrison initially refused to accept any of these proposals, but the Board was equally stubborn. In August 1765, a panel of six experts gathered at Harrison's house in London and examined the watch. One week later, they were satisfied that the disclosure was complete and had signed a certificate to this effect. The Board then insisted that the four timekeepers should be handed over to them, and asked Harrison to recommend someone who could copy H4. Reluctantly, he recommended Larcum Kendall, a leading watchmaker who had probably contributed to the construction of H4, and finally received the first half of the longitude prize. Harrison began working on his H5 while the H4 testing was conducted, with H4 being effectively held hostage by the Board. After three years he had had enough; "I cannot help thinking," he wrote to the board, "but I am extremely ill used by gentlemen who I

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might have expected different treatment from... I hope I am the first, and for my country's sake, shall be the last that suffers from pinning my faith on an English Act of Parliament." and he decided to enlist the aid of King George III. He obtained an audience by the King, who was extremely annoyed with the Board. King George tested H5 himself at the palace and after ten weeks of daily observations between May and July in 1772, found it to be accurate to within one third of one second per day. King George then advised Harrison to petition Parliament for the full prize after threatening to appear in person to dress them down. In 1773, when he was 80 years old, Harrison received a monetary award in the amount of £8,750 from Parliament for his achievements, but he never received the official award (which was never awarded to anyone). He was to survive for just three more years. In total, Harrison received £23,065 for his work on chronometers. He received £4,315 in increments from the Board of Longitude for his work, £10,000 as an interim payment for H4 in 1765 and £8,750 from Parliament in 1773. This gave him a reasonable income for most of his life (equivalent to roughly £45,000 per year in 2007, though all his costs, such as materials and subcontracting work to other horologists, had to come out of this). He became the equivalent of a multi-millionaire (in today's terms) in the final decade of his life. Initially, the cost of these chronometers was quite high (roughly 30% of a ship's cost). However, over time, the costs dropped to between £25 and £100 (half a year's to two years' salary for a skilled worker) in the early 19th century. Many historians point to relatively low production volumes over time as evidence that the chronometers were not widely used. However, Landes points out that the chronometers lasted for decades and did not need to be replaced frequently — indeed the number of makers of marine chronometers reduced over time due to the ease in supplying the demand even as the merchant marine expanded. As well, many merchant mariners would make do with a deck chronometer at half the price. These were not as accurate as the boxed marine chronometer but were adequate for many.

2.2 Process Description The purpose of a chronometer is to keep the time of a known fixed location, for example Greenwich, England, which can subsequently serve as a reference point for determining the ship's position. By comparing local high noon to the chronometer's time, a navigator could use the time difference between the two locations to determine the ship's present longitude. Since the Earth rotates at a steady rate of 360° per day, or 15° per hour (in sidereal time), there is a direct relationship between time and longitude. If the navigator knew the time at a fixed reference point when some event occurred at his location, the difference between that time and his apparent local time would give him his position relative to the fixed location. For example, knowing the time at a reference location when the apparent local time the sun reached its highest point in the sky (local noon) would yield the location.

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Hence suppose the local time at the place where you want to calculate the longitude is 4 PM and the time at Greenwich (the time your chronometer displays) indicates noon, then the longitude of the place can be calculated as follows :Local Time at Place X – 4:00 PM Local Time at Greenwich – 12:00 PM Time Difference – 4 Hours Now, 1 Hr - 15° 4 Hr – 4 X 15° = 60° Since the place is ahead in time it is east of Greenwich. Hence it lie’s 60° east of Greenwich. Hence Greenwich soon became a standard and the Greenwich Mean Time (GMT) came into use. Greenwich Mean Time (GMT) is a term referring to mean solar time at the Royal Observatory in Greenwich, London. One might wonder when the mariner is in the middle of the ocean how does he/she find out the local time of the place. The mariner in such a case compares local noon time (when the sun is directly overhead) to GMT time.

3. CONCLUSION The objectives of my project were to • To provide a clear understanding of what a chronometer is. • To let the reader get a glimpse into the life of John Harrison and the various hardships he faced. • To give the reader the different types of chronometers (invented by Harrison) and their physical description. • To explain ‘how to use a chronometer’ A detailed description of what a chronometer was given. Also the ‘story like representation’ gave the reader some idea on John Harrison’s life and the various difficulties he had with his inventions and the Longitude Board. The various types of chronometers invented by John Harrison (H1, H2, H3, and H4) were also given with pictures. However the exact mechanisms of working of each of these types were not given because it was difficult obtaining information on the mechanism. Lastly how a chronometer is used was explained with a numerical example.

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4. REFERENCES 1. http://en.wikipedia.org/wiki/Marine_chronometer 2. http://hubpages.com/hub/Marine-Chronometer 3. http://www.manitobamuseum.ca/sg_marine.html 4. http://clubswanmagazine.nautorgroup.com/clubswanmagazine/imgsriviste/11_17_2 008_13_19_32.pdf 5. http://www.watches.3345.com.au/cyclopedia/watches_special-marine_chronohistory.html 6. http://photos.revolution-press.com/Jack%20Folder/escapeweb/chronometer.jpg 7. http://en.wikipedia.org/wiki/History_of_longitude 8. http://westernisle.ca/History/Nav1.html 9. http://en.wikipedia.org/wiki/Navigation 10. Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time By Dava Sobel

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