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Old 05-06-2005, 14:17   #1
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Ever wonder ... ?

Why is a nautical mile per hour is called a "knot"?
Before the invention of an accurate chronometer, ship captains were able to determine latitude (north south position) accurately, by knowing the date and the position of the stars and sun. But longitude (east west position) could not be determined from the heavenly bodies, without knowing the time to a high degree of precision.
They used "dead reckoning" to compute where they were. They figured this out from their direction and speed. The compass gave them their direction, but they needed a way of telling how fast they were going.
A simple device, a "knot meter" (log), could tell them just how fast they were going. It consisted of a blown glass container of sand (an hour glass) that ran for about a half minute, and a rope with a small bucket shaped piece of wood (log) on the end. The rope was held at a certain position on the boat, and the bucket was dropped into the sea. When the starting point of the rope began to run through the fingers of the deck hand, the hour glass was turned over. The deck hand counted the number of "knots" that went through his hand until the sand ran out. The knots were tied in a spacing that corresponded to one nautical mile per hour.
Simple and effective, within the technology of the day. But the accumulated inaccuracy of this dead reckoning method is one reason why so many ships wrecked. The accurate stable chronometer made the determination on longitude possible, and was one of the greatest inventions of its time.
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Old 05-06-2005, 16:16   #2
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Patrick O'Brien novels

The subject of using a "Log" to determine boat speed was thoroughly recounted by Patrick O'Brian in his novels. Jack Aubrey, of "Master and Commander" fame used this method in all 21 books. I enthusiastically recommend them. Once you start, you will find it difficult to put them down. They take a little getting used to, but stick with them.

I recently read a book entitled "1421" which related how the Chinese discovered America. I could be wrong, but I think they were the first to determine longitude. Cannot remember how they did it.

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Old 05-06-2005, 20:38   #3
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Veering off on chronometers…

There is an interesting, but brief, article about John Harrison's first reliable "sea chronometer" at solarnavigator. Note Harrison's abandonment of a "large counterweight equals smoothness" approach for a solution centered on low mass, springwork, & constant tension. Quite elegant.

Harrison's H4 made possible Cook's Pacific exploration. Cook used a knock-off of Harrison's H4, reverse-engineered from his original and built to a price-point to make its use on all English ships feasable (some 200 English pounds, compared to 500 for a genuine Harrison). Harrison feared this scenario all along (were there even patents then?), and was reluctant to hand over his invention to the Royal Observatory (and into the hands of his chief competitor, Willaim Halley, of comet fame) to collect his 20,000-pound reward, the so-called "Longitude Prize," offered by the Crown for just such a device.

Interestingly, it was a version of this same basic timepiece that Lewis and Clark used on their overland expedition.

Well, that's the little bit I know. Any search engine yields some very informative sites which detail the entire history, if one is inclined to such arcana.

Thanks, Gord: you just gave me my afternoon's reading.
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Old 05-06-2005, 20:47   #4
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InRe: Chinese Explorers

I watched a whole hour program (PBS— I'm suspicious immediately) that explored this idea, including lengthy interviews with the author, and evaluations by Chinese historians, which put the whole hypothesis set forth in 1421 in a doubtful light.

It seems the idea is based on fragmentary and anecdotal evidence, and is highly speculative, at best.
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