The use of the bell and hourglass to mark the passing of time on board ships was discussed in the last article.

Alongside the bell in ships of the past was placed an hour glass - usually keeping track of half an hour of time with a measure of sand running from the top to the bottom of the glass.

This marked the passage of time and it was an important part of watch-keeping duties to maintain a keen eye on the hour glass, turn it smartly upon its emptying, and mark the time by ringing the bell.

With the ship's speed known by casting a log line and an accurate measure of time taken over 24 hours, the distance covered for the day was able to be calculated.

This was an important part of navigation, especially in the days of dead reckoning alone being the main means of knowing your whereabouts.

With the invention of mechanical clocks the hourglass might have been expected to become obsolete, but it remained alongside the bell as essential equipment for quite some years - well into the 19th century in fact. This was because perfecting a sea-going clock was a frustrating and elusive matter.

The lack of accurate timekeeping was unfortunate for a number of reasons.

Dead reckoning navigation was carried out by compass, speed and time monitoring, and gave a distance that could be entered upon a chart, with a position being thus roughly known. However, it was notoriously inaccurate, due to the lack of precision in all three parts of the equation.

A calculation of latitude - how far down the globe you were as measured from the poles - was able to be determined by measuring the angle between the sun and the horizon at noon, and as long as you knew the date, you could work out that placement.

The angle was at first measured by a staff, and eventually by a sextant.

Longitude was a different matter - it was a man-made determination rather than a matter of observing nature.

Longitude measured how far around the globe you were, and eventually all navigators took their 'starting mark', or meridian, from Greenwich in London.

If you knew the time at your position, and also the time at the meridian, then you could work out your longitude. Since the globe rotates 360 degrees in 24 hours, then an hour of time difference meant 15 degrees.

Here was where time measurement was important, as even a few minutes of inaccuracy could mean quite an amount of distance.

The early mechanical contraptions made to measure time did so to a considerable degree of inaccuracy. By the 16th century mechanical timepieces were not uncommon in Europe, but many of them lacked accuracy, and almost all needed a stable surface on which to run.

They mostly used a pendulum, invented, some say by Galileo, and some say by Christiaan Huygens.

The early pocket watches, apart from being expensive, were notoriously inaccurate - up to 15 minutes worth of time a day could be lost or gained. But a sea-going clock was essential to good navigation - and the cost as shipping became more sophisticated and more widespread - was important.

In November 1758, Admiral Keppel's squadron lost their lead ship when it drove ashore and the rest of the squadron was only just saved. At the time the navigation put the ships 350 miles from land.

In 1760 the British warship Ramillies was trapped inside a bay on the Devon coast after her captain mistook his landfall. Eventually she was driven ashore, and all but 27 of her 800 crew drowned. Such incidents - only two years apart - were common.

While early mechanical clocks were reasonably accurate, they were not so at sea. The pendulums of the clocks was thrown off by ship's movement and as the ship rolled, so did the pendulum either speed up or run fast.

Pressure and temperature also had effects, with metal parts contracting or expanding.

The accepted method of coping with the lack of knowledge as far as longitude went was to run along a determined latitude, but of course that had its limits. What was needed was a good timepiece.

It is hard to realise now the desperation felt by navigators who could not realise accurately their whereabouts.

The problem of determining longitude even drove governments to offer cash prizes for a solution. And eventually one was found.

A British inventor by the name of John Harrison, over a period of 40 years, invented and perfected the sea-going chronometer.

He was an ingenious man indeed - the first of his clocks were built of wood, to avoid the problems associated with metal and oil. A complicated method of springs, gears and wheels did away with the pendulum.

Therefore perhaps it's the case that this one man - John Harrison - made possible supremely accurate navigation and opened the world up to the great sea navigators.

Footnote: An invaluable reference to this fascinating subject - which this article has also used as a source - is 'Longitude', or its even better version 'The Illustrated Longitude', by Dava Sobel and William JH Andrews. The book (ISBN 1-85702-714-0) discusses the whole problem of keeping time at sea, and the unusual life of maritime clockmaker John Harrison.

LEUT Tom Lewis