## Saturday, 12 December 2015

### Keeping a log

If you want to understand a log, the place to begin is with the nautical mile, which is exactly 1000 fathoms, 6000 feet, or the distance covered by one minute of latitude. Once you understand that simple relationship, most of the rest of the calculations to do with navigation become easy indeed.

The problem with sailing away from land is that you are never quite sure what you are going to bump into, just over the horizon, because you don't know quite how far you have sailed, and while you can use sightings on the sun and the stars to work out how far you are north or south, east and west is another matter, and to sort this, you need a very accurate chronometer.

At the equator, a degree of longitude east or west is also 60 nautical miles, away from the equator it is less. If you are using a timepiece to work out where you are, the skies appear to roll past at a steady 15° each hour, which means a degree every four minutes. If your timepiece is wrong by just one minute, that is a quarter of a degree, 15 nautical miles, far enough to make the difference between safe at sea and dying of thirst in a desert.

This is why most early navigators would get to the right latitude, and then sail along carefully, until they hit their target port. One advantage of doing this was that all the calculations that are needed as you angle across the latitudes and longitudes disappear, as if you were sailing on a flat surface, rather than across a huge sphere. In fact, sailors called this 'plane sailing', though these days, we are more likely to say "it's all plain sailing from here". Incidentally, the people who praise William Bligh for his epic journey across the sea after the mutiny on the Bounty should look at the map again: Bligh was a good navigator, and made his task easier by relying on plane sailing.

When Dutch ships sailed from Europe to the East Indies, they rounded Africa, sailed east across the Indian Ocean, and then turned north, hopefully before they hit Australia, but many of them missed, and so the Great South Land became New Holland for quite a few years, due to a certain amount of bump-and-grind inadvertent discovery by Dutch seafarers who had slightly lost track of how far east they had sailed.

To avoid this sort of problem, navigators used all sorts of tricks to work out where they were, and how fast they were going. One way was to drop a log of wood over the side at a fixed point on the ship's side, and time how long it took for a second point on the vessel to reach the log.

This wasted logs, and was unreliable, so before long, the log was dropped with a line attached to it, to see how much of the line paid out in 30 seconds or 60 seconds, measured by a sand-glass.

The old dropped log was not entirely forgotten, because English sailors called this a Dutchman's log, but they soon found a better design to use.

 Jeffery Walker's patent log rotator
If you multiplied the length of the line that went over the side by the correct factor of 120 or 60 (for 30 seconds or 60 seconds — you work out why!), you knew how far you would go in one hour at the same speed.

Then somebody had a clever idea: put knots in the line, and count them as they go through your hand. If these knots are 1/120 of a nautical mile (50 feet) apart, then each knot paid out in 30 seconds indicates one nautical mile an hour, while knots at 100 foot intervals are used for a sandglass period of 60 seconds.

Later, better ways were found of getting the measurement, like Walker's patent log rotator, shown on the left,

Wind and water speeds are still measured in knots (not 'knots per hour') because of this practice. Later, the 'log' grew much fancier, and even became a small propeller in a tube set into the hull, which drove a gear system to provide sea mile data.

No matter how they were obtained, the results were, of course, written in a 'log book', and that term came to apply to any record, such as the record of people accessing a computer system.

And that is why, today, we 'log on' to a computer system, though few of us spare much thought, as we do, of the horny-handed old salts who heaved lumps of wood over the side of old sailing ships.

Bloggers think about their origins even less.