Friday, October 08, 2004

Given that I've become mildly obsessed with the Bush-Kerry race, I feel the need to do a non-politics related post for my own sanity.

Lately, I've been reading Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time, by Dava Sobel, an amusing biography of John Harrison, a now-forgotten clockmaker who constructed a series of elaborate clocks designed to keep accurate time at sea.

Back in the day (mid-to-late 18th century) the holy grail of science was the question of finding a method for a ship at sea to locate its position. One can determine latitude simply by the length of the day or the trajectory of the sun; determining longitude is more tricky. There was no practical way of doing this based on the sky, and, as a result, ships constantly got lost.

Besides the large number of ships lost to navigational problems, much was also lost to piracy: ships tended to stick to known navigational routes, which consisted of keeping the same latitude in the trip across the oceans; pirates had an easy time preying on these routes; and one must remember that most empires of the time made their earnings not so much from taxes but from trans-oceanic trade, so the losses were immense.

The problem had assumed mythical proportions with time. In Gulliver's travels, for example, when Lemuel Gulliver imagines himself as one of the immortals, he wonders about seeing such things as "the discovery of the longitude, the perpetual motion, the universal medicine, and many other great inventions brought to the utmost perfection."

One could figure out longitude by knowing the time back in the home port and comparing it with the local time; but there were no clocks accurate enough to keep time at sea. The constant shaking set them off; the changes in temperature, dryness, wetness caused metals to expand and contract and keep time at a different pace; moreover, the best clocks at a time erred by as much as 15 minutes per day. By contrast, to be useful for telling longitude on a trans-atlantic journey, a clock could not err by more than 3 seconds per day.

Harrison was the man who solved the puzzle, devoting his entire life to constructing an intricate series of clocks which could keep time at sea with sufficient accuracy. He made clocks out of different metals such that the expansion of any one metal at a higher temperature would be balanced by the natural contraction of another; he spent his entire life perfecting the idea into a workable, mass-producible clock.

The most intriguing part of the book is dedicated to the harebrained schemes for computing longitude that existed before Harrison came along:
...the most colorful of the offbeat approaches was the wounded dog theory, put forth in 1687. It was predicated on a quack cure called the powder of sympathy. This miraculous powder, discovered in southern France by the dashing Sir Kenelm Digby, could purportedly heal at a distance. All one had to do to unleash its magic was to apply to an article from the ailing person. A bit of bandage from a wound, for example, when sprinkled with powder of sympathy, would hasten the closing of that wound. Unfortunately, the cure was not painless and Sir Kenelm was rumored to have made his patients jump by powdering - for medicinal purposes - the knives that had cut them, or by dipping their dressings into a solution of the powder.

The daft idea to apply Digby's powder to the longitude problem follows naturally enough to the prepared mind: Send aboard a wounded dog as a ship sets sail. Leave ashore a trusted individual to dip the dog's bandage into the sympathy solution every day at noon. The dog would perforce yelp in reaction, and thereby provide the captain a time cue. The dog's cry would mean, "the Sun is upon the Meridian in London." The captain would then compare that hour to the local time on ship and figure the longitude accordingly. One had to hope, of course, that the powder really held the power to be felt many thousand leagues over the sea , and yet - and this is very important - fail to heal the telltale wound over the course of several months. (Some historians suggest that the dog might have had to be injured more than once on a major voyage.)


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