The Mariners’ Instruments
- Extra Reading
Next lecture in the series is Franklin and the North West Passage
Visit www.seabritain2005.com for information on festivities and events throughout 2005 and beyond. SeaBritrain2005 celebrates the ways in which the sea touches all of our lives.
I'm going to talk mostly about the navigation that developed in North West Europe and the Mediterranean. This has a convoluted history. I am going to attempt to cover the period from medieval times up until the end of the 18th century.
There are many paintings showing English ships in a gale. They typify the terrors of navigating sailing ships, sometimes on a lee shore or sometimes on the gales that they encounter in oceans around the world. There comes a time when ships' navigating officers have to make vital decisions. It can be when they are making a landfall after a long protracted voyage, the sky overcast, they've not been able to get a fix for the ship. There is much discussion amongst them all as to exactly where the ship is and what course they should steer. Samuel Pepys, who went on a voyage to Tangiers in 1683, to supervise the demolition of the mall there, was absolutely amazed at the incompetence of most of the navigators in the naval ships he sailed with. Pepys says:
It is most plain from the confusion that these people are in, in how to make good their reckoning, even each man's with itself, and the nonsensical arguments they would make use of to do it, and disorder they are in about it, that it is by God's almighty providence and great chance and the wideness of the sea that there are not a great many more misfortunes and ill chances in navigation than there are.
So Pepys was not desperately impressed with the technique and skills of the 17th century navigators in the Royal Navy.
If they make the wrong decision, you can make the supremely unfortunate mistake of arriving rather suddenly amongst the rocks and the breakers. So seamen have always been conscious that they need all the assistance that they can develop and find. I suspect that you will have guessed that the earliest navigating instrument was the sounding lead. As ships approach the Channel, perhaps on a voyage from Bordeaux or the Portugal ports, it was the 100 fathom line that they would search for. The lead itself, which was armed with tallow, was lowered from the side of the ship, the ship would heave-to, they would lower the line down and discover how deep the water was. If they hadn't reached the 100 fathom line on the continental shelf which protrudes out from the English Channel and surrounds the North West European shores, they would sail on a little further until they picked up the sea bottom, and then the tallow on the bottom of the lead would pick up from the sea bed its character, either fine sand or mud and ooze, and experienced seamen would be able to tell from this exactly which part of the Channel's mouth that they were in. So the sounding lead was a commonsense instrument. It was used in very deep water, and then as you got into shallow water, you used the hand lead, which had a seven pound weight on the bottom of it, and markings on the line.. These were in existence we know as early as the 1620s, where you used bits of cloth and lengths of rope with knots in it. I know that white was five fathoms, and that red was seven fathoms – and I know this because I was taught that there are five letters in white and there are seven in crimson. Those marks were established three or four hundred years ago.
The sounding lead was a fairly obvious device to make use of but the most important, perhaps the most important, invention for seamen was the magnetic compass. There is a compass in the National Maritime Museum collection, date unknown, but we think possibly middle of the 16th Century, perhaps from the 1580s. It's in turned ivory. The motion of the vessel doesn't upset the steadiness of the card under which the magnetic needle is suspended. The first record of a magnetic compass in Western waters was by an English monk, Alexander Neckham, in 1187. He was travelling to somewhere on the Continent, and in crossing the Channel he noticed that seamen were using this rather strange device when the sky was overcast and they lost a sense of their direction.
Vincent Bouvey in 1240 described in graphic detail how this magic trick was performed. The magnetic needle was stroked with a lodestone, a naturally occurring magnetic ore, the needle was magnetised, and then thrust, at right angles, through a little piece of straw and floated in a bowl of water. Then they would whirl the lodestone around the bowl, and the needle would be dragged round with it by the magnetic attraction. The lodestone would be snatched away and eventually, when the needle settled, in theory it pointed to the north. Now of course this could be checked when the skies were clear and you could see the Pole Star, but in overcast conditions, obviously this was a very useful device.
The magnetic compass was a tremendously useful addition to the navigator's armoury. No one is really sure exactly when it was pivoted on a needle in the bottom of a bowl, and when the compass fly or card was stuck to the needle and marked with the 32 points of the compass, but we think it was in the middle of the 13 th century, round about 1250 or so. We know the Chinese developed a similar instrument round about 1100. Obviously this instrument was being developed in Europe at much the same time.
The magnetic compass made a tremendous difference to navigation. Mediterranean sailors, since classical times, have been sailing the length and breadth of the Mediterranean, and out into the Atlantic. Roman sailors came to our shores. In some ways the Mediterranean was an ideal cradle for navigation to develop, because it's enclosed almost entirely by land. It is not subject to currents as the sea is around the North West European shores. The water is very deep, and the cliffs are steep too. The winds tend to be seasonal and steady, and it was in the Mediterranean that the galleys, vessels which went along at a steady pace, were developed. So with a combination of magnetic compass courses, which gives you direction, and the steady speed of the ship, which would give you the distance over a period of time, a very clear image of the Mediterranean's coastline was built up over centuries.
At about the same time that the compass was introduced, the Portulan charts came along. Originally they were an illustration of the sailing directions that seamen used to help them find their way about from cape to cape and from coast to coast. They are reasonably accurate as to the delineation of the coast, but the coastline of the Mediterranean in these charts is about 11 degrees out because there is a geographical pole on our Earth and a magnetic pole, and the two do not coincide. The magnetic pole moves around on an annual basis, and this causes the magnetic field of the Earth to alter annually. In the Mediterranean at this time the error of the compass was about 11 degrees, but this doesn't make any difference. If you're using a compass with an 11 degree error, and you've got a chart with an 11 degree error in it, the two match up very nicely.
When artefacts began to appear from the Mary Rose, which we know sank in 1545, I was particularly excited because I thought we would find some navigational instruments in it. Three compasses were recovered from Mary Rose. I have a picture of one. You can see it's glazed. It has a glass top to it to protect the card from being influenced by the wind and so on, and the circular piece of wood to the right with the pin you can see stuck through the bottom, which is the pivot for the compass card, that plugs into the bottom of the tubular item on the left. There was no magnetic needle or card in the compass. It was discovered in a sea chest in one of the cabins of the Mary Rose, the so-called pilot's cabin, because other navigational instruments were found there also. When it was freeze dried and reassembled, this compass was totally complete apart from its card and the magnetic needle. It was contained in a box with a sliding lid, with finger grips that pulled the lid back. Unfortunately, the original putty that held the glass in disintegrated during the conservation process, but that particular compass, it surprised me, because it was almost identical to a magnetic compass. If you look at a French compass from about 1800, you can see that in about 250, 300 years, there was be no improvement in either the performance of the compass or the techniques by which it was built. The improvement of magnetic compasses really only got under way in the mid part of the 18th century, but it's a 19th century story and perhaps a story for another day.
When the compass was used on a ship, they would have one by the man who was steering the ship with either the tiller or the whiplash, which was fixed to the end of the tiller, so that it could be used by night and so that it could be protected from spray and so on. It was placed in a small cupboard known as a binnacle, or a habitacle. The earliest record in English ships for habitacles is about 1410. One was recovered from a bask whaler on the Labrador coast, and the ship we know sank in 1565. It was a simple cupboard, with a sliding panel on the front and the compass protected there inside it.
The other instruments that were early were sandglasses, which we know were at sea in the middle of the 13 th century. A sandglass was recovered from an English warship that sank in 1703, but in fact it's no different from the sandglasses recovered from Mary Rose 200 odd years earlier. It is made of two glass vials which were filled with very fine sand. It was timed against another sandglass, a standard sandglass, and then sealed and bound and put in a wooden cage to protect it. We know that they carried different sizes of sandglass in ships because not only was the day regulated for the working of the ships, watches were controlled by the sandglass. There would be a sandglass by the helmsman and he would turn it at the end of a half an hour. The smaller sandglasses ran for half an hour. But so that he didn't warn the bell as it were, the officer of the watch would have a sandglass that perhaps ran for an hour, so the man at the wheel or the tiller would turn the sandglass at half an hour, strike one bell at the beginning of his watch, and when he'd turned it twice, he'd strike two bells, and this is the process of watches in ships. Seamen served four hours on, and in the early days, four hours off, and they struck a bell every half an hour. So four bells, you knew you were half way through the watch, and so on, eight bells was the end of the watch, and you would have turned the glass eight times. They had a series of sandglasses just to check against each other.
It regulated time in the ship, but it also regulated the time that you were sailing on a particular course, so it was a very essential tool to the mariner. It was a combination of direction with the compass and timing, timing the length of time you were on a course, with a sandglass were obviously essential in the very early days.
Also amongst the items recovered from this wreck of 1703 were little slates, which were used for recording the day's work, recording the speed of the ship through the water, the course it was steering and wind condition etcs. Slates like these have been found in a number of ships from that period. I have to admit that when I was at sea in the 'fifties and 'sixties, the company I was with still used slates to record the progress of the ship in the wheel house. We wrote with a slate pencil, the idea being that if there was some dreadful catastrophe, the ship's master could decide what had happened and the slate could be adjusted, whereas if you'd written it down in ink or somewhere in a book, you were stuck with it! That was the anecdote, but I suspect that's all long gone now.
Judging your distance and the speed at which a ship was sailing – the two are obviously interconnected – was a difficult thing to do. The Spanish and the Portuguese judged the speed of their ships by the amount of sail the ship was carrying, and they could translate that into distance. At one time, it was also known that you could mark a length on the gunnel of the ship and that length was proportional to a mile. You would toss in a chip of wood or something, or you would see a fleck of foam, and you would time it past this length on the gunnel, and the time it took was proportional to an hour that this length was on the gunnel, and then you could turn that into the distance the ship was sailing in an hour. Well, this is all very prone to error, obviously. The English log was invented by a seaman in England, we know now, before the Mary Rose sank. Prior to the work on Mary Rose, it was thought that the English log wasn't introduced until the latter part of the 16th century, where William Bourne describes it in his book "Navigational Text".
The way it was used, you threw a little triangular piece of wood over the stern of the ship, and waited until it had got clear of the eddies, and then you turned a half minute sandglass, and allowed the line to run out over the stern of the ship. When the sandglass ran out in half a minute, you stopped the line, and then bringing it in across your chest, which is a fathom, six feet, you measured how much line had run out in half a minute, and then half a minute is of an hour what that length of line is to the distance the ship has run. While measuring a wet line like this over your chest, someone had the bright idea that it would be a useful to put knots in the line at certain distances. The trouble with that is that to get the knots the correct distance apart, you have to know the size of the Earth, and no one was really sure of that until the mid-eighteenth century, although the Greeks had in fact worked it out very accurately many thousands of years before. So seamen tended to have the knots too close together. The correct distance is 51 feet and that equates to sea mile of 6,080 feet. But seamen, being the cautious people they are, preferred the reckoning to be ahead of the ship, so when they were reckoning how far the ship had sailed, they would much prefer that reckoning to be ahead of where the ship actually was. If it's the other way round, you think you're safely in the ocean and the ship is ahead of you, so they always made the knot distances shorter than needs be. So they'd make them 45 feet. The sandglasses they used in the 19th century, just to be on the safe side, instead of running for 30 seconds, they ran for 28 seconds, and so they were always underestimating the speed and the distance that the ship had run – or overestimating rather, so that the ship was behind the reckoning.
All of these things went together to form what we call today dead reckoning. You used all the information that was possible, your knowledge of tides and currents. This knowledge wasn't extensive out in the oceans, but around the coasts, shipmasters learnt, master to apprentice, over hundreds of years, the courses between capes, times of high and low water. It was known that the tides arrived later by about 48 minutes each day. In fact, the ports around European coasts were given what we know call an establishment, but seamen thought of the compass card, its 32 points – north equated to midnight, east was six, south was 12 hours, and because there are 32 points of the compass, if you divide that into 24 hours, it works out at about 45 minutes. By knowing the establishment of a port at full moon, which is when the tides are highest, in spring tides, that fall and change, they could work out from the compass card, knowing the age of the moon, which they'd look up in their little almanacs, they could work out what time high water would occur at the various ports around the UK.
That is quite difficult to explain, but another instrument found in the Mary Rose was a little circular disc, which has on it the 32 points of the compass. At the north point, it has a peg. At the south point, it also has a little distinguishing mark. It's quite crudely done. I think that this was a tidal computer. The master of the ship, or one of the sailors, would know that if, at full and change, high tides were at midnight, he would know that after three day, he could work back three points on the compass and it would give him the time. So it's a little handy pocket computer.
There were dividers found on Mary Rose. It's obviously a standard instrument that seamen use now on picking off distances on charts, although it's all done for you electronically these days. But a pair of dividers, or compasses as they were known, was found on Mary Rose, and it has given us cause to think that perhaps there were charts on Mary Rose, although when she sank, British seamen, English seamen, were not too familiar with charts. They were being developed in the Mediterranean, and then with the Portuguese and the Spaniards, who were sailing off to the New World and out to the Far East. British seafarers were not much taken by this, and reckoned they could keep a better reckoning on a travis board, which was something on which you could plot a ship's course and its distance.
There may have been charts in the Mary Rose, but we shall never know because velum charts are not able to survive in sea water for very long. One of the earliest English charts of any significance was by Thomas Hood, and it was drawn in 1598. It shows the Western approaches. It shows the continental shelf with its soundings and the 100 fathom line. It incorporates a latitude scale, and it also has tidal information. It tells you the times of high water at the various ports around North West Europe. Of course that's most important information because at a place like Bristol, the difference between high and low water can be 35 feet, so if you're sailing in with quite a large ship, you want to be sure there's enough water beneath the keel of your vessel.
Well, this was coastal navigation, and I suppose we call it pilotage now. Seamen travelling across short seas from headland to headland, cape to cape. In the early sailing directions, they didn't express it in miles, they expressed it in kennings, in other words, how far you could see from one cape to the next, and a kenning in English terms was about 20 miles. The word 'kenning' is still used up in the North – do you ken John Peel? It's knowing the distance, basically.
Prince Henry the Navigator was the third son of King John II of Portugal. He married an English princess. Prince Henry was very interested in getting to the riches of the Far East by sailing around the Islamic Empire, which had control of Africa and the Levant. So he sent his seamen exploring off the African coast. They progressed further and further down this coast. Keeping track of how far they had gone was a difficult proposition, but they were taught this method of using the Pole Star. If you stand at the North Pole, the Pole Star is directly overhead, and as the Earth rotates, the Pole Star stays overhead. If you go down to the horizon, the Pole Star is right on the horizon. Anywhere in between the North Pole and the Equator, the Pole Star actually corresponds to the height you are above the Equator, in other words, your latitude. So by measuring the height of the Pole Star above the horizon, this gives you your latitude or your distance north of the Equator. It's quite a simple sum.
Instruments were devised for seamen to be able to measure this simple altitude observation. The first instrument they used was a quadrant. These were astronomical instruments adapted for use for seamen. There was one inherent failure, and that was its use of a plumb ball. If you're on a ship, where it's windy and the ship is rolling and pitching, obviously it takes a very careful eye to get the altitude correctly. Seamen were taught to mark the altitude of their port which they were departing from, and then as they sailed south, they'd record the height of the Pole Star when they were off headlands or discovered islands, and by doing this, they could mark it on the quadrant. It was the Portuguese who discovered the wind systems in the North Atlantic: they found they couldn't sail back up the coastline again, they had to sail out into the Atlantic, where the winds had a circular, clockwise rotation. They would sail north again until the Pole Star equated to the mark they had put on their quadrant for their home port, and then they would turn east and then sail towards the coast. This was known as altura sailing, the height of the Pole Star.
The quadrant was not an ideal instrument, and they adapted the astronomer's astrolabe. The earliest types had a solid disc with an alidade or ruler on it with two sights with pinholes in it. The pinholes and the plates in which they are were moved closer to the centre point because it's very difficult to get the sun's rays to go into one hole and strike the other hole. It had a scale on which they could read the height. They discovered that the solid discs - which were quite large, some of them two feet across – would blow around in the wind so you wouldn't get a very steady reading, so gradually they developed into cast brass instruments.
When I joined the Maritime Museum only about six of these were known, but I soon realised that if we were going to learn any more about the earlier instruments of navigation, they would be all coming up from wrecks. So I attended many underwater archaeological conferences, and my spies then were around the world and they used to write to me and give me information on these instruments. Eventually I had a total of over 60. I published a book on it, so if you are interested in mariners' astrolabes, that's the book to get!
Sailing beyond the Equator, when the Portuguese got down beyond the Equator, of course the Pole Star dips below the horizon and you can no longer use it, but the astronomers that Prince Henry the Navigator could call upon produced tables for the Sun so that you could use the Sun to find your latitude. You can only measure the Sun as it crosses your meridian at noon each day wherever you happen to be; iin other words, when it reaches its highest point. You also have to correct it because of its seasonal changes. In the summer, in the northern hemisphere, the Sun is higher in the sky, and so on. So tables were produced which could correct this but seamen, being the simple souls they weere, some of them were much more easily satisfied with a diagram which told them which way to apply the declination. So if the Sun has got a northerly declination and you're in the northern hemisphere, north of the declination, it tells you which way to apply the correction, and then all the different cases of being in the southern hemisphere and the Sun is in the northern hemisphere because it's a northern hemisphere summer. All those different instances were recorded in a diagram, which made it very much easier for the average navigator.
Another astronomical instrument that was developed for seamen was the cross-staff, a simple instrument, that was quite difficult to use. You put one end of the staff to your eye, and the cross on it you slid up and down the staff until the top of it was just covering the Pole Star, or the body you're observing, and the lower end of it was just touching the horizon. Well, this is quite difficult to do, and in fact you can't measure angles over about 50 degrees with it, but it was much used at sea. When you were using it with the Sun of course, gazing towards the Sun is very difficult, and you would put a shade on the end of the cross to protect your eye. It was recorded that most old seafarers at this time were blind in one eye or had impaired vision because of staring at the Sun to get its altitude.
Well, using these methods, the new declination tables, altitudes of the Pole Star, seamen and geographers were able to compile very much more advanced charts. By 1529 charts appeared that were quite accurate for latitude Most of the latitudes were within an acceptable tolerance. The longitudes were not yet there.
The charts were often drawn on a whole skin of velum: a cross was put on it, and then from the centre point, you drew a circle, and the circle was marked by little compass roses that go round, and from each of those compass roses, you drew the 32 points of the compass, and you drew them also from the centre of the chart. As a consequence, these charts were covered in a maze of rums or lines from which you could pick your course when sailing from point A to point B.
During the latter part of the 16th century, Captain John Davis invented the backstaff, which helped you preserve your eyesight. Instead of gazing directly at the Sun, you stood with your back to it, and then the two arcs together made 90 degrees and you had a sight vane and a shade vane. The shadow vane threw a shadow onto the horizon vane, which you can see has got a slit in it, and by peering through the sight vane at the horizon, you made that coincide with the shadow cast by the shade vane from the Sun. You could adjust this by moving the sight vane up and down the large scale, and when you felt all was right, you read off the sum of the two arcs and this gave you your altitude of the Sun. You might get an answer to within about 15 or 20 miles with an instrument like that. There's an example in the National Maritime Museum at Greenwich.
Of course at this time, they were developing pilot books. They were known as rutters from the French routier, or wagoners because in the 17th Century and the late 16th Century, the Dutch produced them, Jansun Wagoner, sailing directions with little pictorial images of what the coastline looked like, so that when you were approaching the coast, you could work out, as one of them says, "When as Torbay bears north-northwest from you and that you are about three leagues from the shore, then the land appears in this order." In other words, if you weren't familiar with this part of the coastline, you could look it up in a book, and of course pilot books are still used today. All of this information was gradually accumulated over many centuries. The Dutch were pre-eminent in navigation in the 17th Century.
Gunter's rule or scale was developed by one of the professors at Gresham College in the early 17th Century. Edmund Gunter invented the sector, and also a rule which incorporated logarithmic functions. Now, I won't go into logarithmic functions this afternoon, but they're very necessary in working courses and distance to go, and courses and distance that you might have travelled.These logarithmic functions were placed on a ruler, rather like a slide rule, and you used it with a pair of dividers, and then by comparing lengths on it, you could work out mathematical problems in which normally multiplication and division would have applied but all you were doing was adding and subtracting the various lengths on the scale.
Well, latitude we've discussed, but longitude was a very much trickier problem. Latitude is fairly simply found. It's very easy delineated. he Equator is midway between north and south. You've got north latitude and south latitude. But longitude, east and west, there was no natural starting point. You could pick any point on the Earth's surface, and say, well, we'll start longitude from here. In fact, this is what most nautical nations did. They used their own observatory as the starting point. So the French based their longitude scales on their charts on Paris, we on London, the Spanish on the Cape Verde Islands, and so on. That wasn't resolved until 1884.
Longitude is in fact the difference of time between places. If it's noon in London, it's five hours earlier in New York, and this is because there's 360 degrees of the Earth's circumference divided by 24 hours, it works out at 15 degrees of longitude equates to one hour. So you can find your longitude if you know the difference of time between a standard place and where your ship is. Now, the theory of it was very easily understood, and in fact Johannes Ferner, as far back as 1514, had suggested using the Moon as a clock. Obviously you couldn't take a clock to sea, but the Moon acts rather like a clock against the starry background. It moves quite rapidly against the star background. n fact, it moves its own diameter in about an hour. If you think of the stars as the hour points on the clock dial, as the Moon moves against it, you could predict where the Moon was going to be at a certain time, at a fixed known place, say Greenwich, and then if you measured the difference at your ship, you could find out what the time was at Greenwich by using the Moon, and then find your local time, wherever the ship was, and by comparing the two, local time and the time at your port of departure, you would know how far east and west you'd sailed.
That was one theory. The other theory was that yes, you could make a clock, and that would be another way. You would take the standard port of departure's time round with you, with a clock that would keep time at sea, but of course the problem with both of these theories was that no one had accurately mapped the star background, no one could predict the Moon's path against the stars with any degree of accuracy, and there wasn't a navigational instrument that could take the measurements, and also there was not sufficient technology to produce a clock that would suffice. Many prizes were offered for finding longitude. It was seen as essential that ships should have safe passage around the world so that their cargos and crews would be safe, and Charles II established the Royal Observatory at Greenwich, in Greenwich Park. The first Astronomer Royal was instructed to find out the positions of the stars so that we can find the so much desired longitude. So this is what the Rev. Flamstead, set about. Something that precipitated the action was the sinking of a squadron of ships running from the Mediterranean under the command of Sir Cloudsley Shovel in 1707. Four of the ships floundered and 2,000 sailors drowned, and there was a national outcry. Something had to be done.
Eventually, the British Parliament offered a prize of £20,000 for anyone who could come up with a practical method of finding longitude at sea. Well, the first part of that equation to be solved was the instrument to take the measurements. John Hadley, the Vice-President at the Royal Society, invented his reflecting octant, which, as described to the Royal Society in 1731, looked very like this. It's an instrument with double reflection. The index arm on the scale can be moved across, and by means of two mirrors, you can see the Sun and the horizon at the same time. It was the first instrument that allowed you to see the sea horizon, which is essential in taking altitudes, and the object you were observing.
This quickly translated into an instrument with a diagonal scale. It had shades to protect your eye. It dispensed with the telescope as too expensive, and they had little pinhole sights. But that kind of instrument, refined, was very quickly put into production and became popular by about the 1750s. In fact, the back-staff continued in use towards the end of the 18th Century because it was so much cheaper than the octant, and most seamen only aspired to finding their latitude at the time.
The business of finding a clock was solved by John Harrison, a carpenter from Lincolnshire, Barrow-upon-Humber. He produced a series of clocks between the late 1720s until 1759. Harrison was remarkable in that he incorporated a tremendous number of new devices. A clock to keep time at sea must not be affected by the temperature, or it must be compensated for temperature fluctuations. At that time, when you oiled a clock, the oil deteriorated and thickened and would slow the clock down. The pivots would wear. So he invented roller bearings and frictionless pinions, and also the most accurate form of clock at this time was the pendulum clock. Well, obviously there are shortcomings of taking a pendulum clock to sea, and he invented a double balance system. He worked away at these clocks, which were enormous and sat in big cases. He had a lot of trouble adjusting them to keep time, but meanwhile, he produced a watch, quite a large watch, about five inches across, which incorporated some of the technical inventions he'd come up with in producing his large clocks, and the watch was tried. First of all, it had to be copied. There were tremendous arguments with the Board of Longitude, who adjudicated on the so-called solutions to the longitude problem. A copy of Harrison 's fourth watch was produced by Lark and Kendall and went to sea with James Cook, where it performed tremendously well.
The other method of finding longitude at sea, the lunar distance method, was solved when Tobias Mayer finally came up with prediction tables. If you're going to use the Moon as a clock, you have to take to sea with you, and have them prepared well in advance, the predicted positions of the Moon perhaps one or two years ahead. This had been the trouble. The Moon is a very irregular passenger around our Earth. Tobias Mayer came up with tables eventually in 1755. He also said, well, he didn't much like Hadley octant – he invented the reflecting circle, which is on the same principles, two reflecting mirrors, but he reckoned it would eliminate all sorts of instrumental errors. The instrument was tried at sea, but was found to be very unwieldy, and so John Campbell, who was testing it, asked John Bird, who made instruments for the Royal Observatory at Greenwich, if he would produce an enlarged Hadley octant to very high standards. The instrument he produced was 20 inches in radius. It weighs something like 17 pounds, so very heavy. In fact, it has a pole to take the weight. With this, he maintained you'd be able to take very satisfactory lunar distances, that is, measuring the distance between the Moon and a fixed star.
The only problem with John Bird's solution with that instrument was the fact that it took him so long to devise the scale. It was all done by hand. Jessie Ramsden came up with a solution to that. He produced a dividing engine. At that time, the Board of Longitude was able to fund worthy causes. Ramsden was given sums of money to perfect this instrument, and also John Harrison. So Ramsden was encouraged, and produced his dividing engine, which in fact was used on all sextants right into the 20th century – not his particular instrument, but instruments like it. With a dividing engine like that, he was able to produce sextants that were much more manageable that went away with Captain Cook on his voyages of exploration.
Because the Board of Longitude had financed the dividing engine, they said it was their property, so that every instrument that Ramsden produced, he had to stamp it with the mark of the anchor of the admiralty. He would put his initials either side of it, Jessie Ramsden. So if you look at a Ramsden sextant, you'll find that all the early ones are stamped like that.
The man that made all these methods practicable was Neville Mascalin, the Astronomer Royal, who had made a voyage to St Helena to watch the transit of Venus in 1761. He'd used Meyer's tables, and said yes, they do work, but they're very unmanageable. Not only is Hadley's octant not quite sufficient, but also it takes about four hours to work out your longitude by using this lunar distance method, and this is well beyond anything that the average seafarer could cope with. Well, Mascalin came back and said, well, I can resolve all these things by pre-working a lot of the work that had to be done in those calculations, and so he produced the nautical almanac for 1767 at the Royal Observatory. It gives the time at noon, and every three hour intervals, so by measuring the distance of the Moon from a star, and by interpreting the distance you got, you could look that distance up and interpret from the tables what the time was at Greenwich where this nautical almanac was produced. So the nautical almanac, first produced was 1767 and annually ever since, transformed maritime navigation.
It's also part of the reason why the Greenwich Meridian was chosen as the international meridian for nought degrees longitude, because in 1884, when the conference was called, 70% of the world's ships were using charts based on the Greenwich Observatory's nautical almanac, and North America, which had been an English colony, also was using those charts. All her time zones were based on the Observatory at Greenwich and the nautical almanac.
Well, we haven't time to go into this in more detail, but the accumulation of all this knowledge, all these instruments, were available to Cook on his famous voyages in the 1760s and 70s, and in a developed sense, the methods that were developed in the middle of the 18th century, with refinements, were available to me in the 1960s. We were still using sextants, nautical almanacs, logarithm tables and so on in the 'Sixties.
© Alan Stimson, Gresham College, 10 October 2005
This event was on Mon, 10 Oct 2005
Gresham College has offered an outstanding education to the public free of charge for over 400 years. Today, Gresham plays an important role in fostering a love of learning and a greater understanding of ourselves and the world around us. Your donation will help to widen our reach and to broaden our audience, allowing more people to benefit from a high-quality education from some of the brightest minds.