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March 6, 2010


Dava Sobel - ISBN - 978080271529X

The search for a means of determining longitude is a millennial quest which reached its culmination in 1759. This is a tour-de-force account of that quest and its solution. She suggests it is arguable that the British Empire owes its existence to the find. Without longitude, "dead reckoning" was the only possibility, and disastrous results awaited most miscalculations. Travel was done within pre-determined latitudes. "Aim here, stay on course and you'll run into something from whence you can determine which way you want to turn to arrive where you intended. This facilitated piracy and bounty hunting. One example offered was a captured Portuguese ship returning from the Americas with a cargo worth half of the net value of the entire English Exchequer at the time (that was millions upon millions of today's dollars.)

Sobel reviews the dilemma in general, and then explores the specifics. Latitude had been reasonably well mastered from before the time of the Phoenicians, but not longitude. Only two reasonable tracks could be pursued: the painfully slow endeavor of comprehensive mapping of the heavens in both hemispheres, and likewise the tracks of the moon (over its 18 year cycle), with complicated coordination of this data into useful equations which could be published in a source book--rather like logarithms in geometry; the other was the creation of a clock which would be accurate within few seconds per day over many months--a feat considered impossible.

The former method was eventually rendered possible on land, with considerable accuracy, but not at sea. On land it had to be determined only once, on a clear night, and recorded as a datum, while at sea there were days, even weeks, when visibility was limited or non-existent, and times when the seas were too rough to accomplish such metrics. Clocks, on the other hand, were then dependent upon pendulums, which for a variety of reasons were non-starters at sea.

From the outset it appears (to me, at least) that the chronograph, as the clock was to be called, was the best solution, but the astronomers were in charge, and did their very best to stay on top. The British Crown had declared a reward of 20,000 pounds--12 million dollars today--to the genius who figured it out. Hundreds of people sought the answer, most of them crackpots.

John Harrison, an autodidact and carpenter by trade, undertook the challenge by first developing a clock with a pendulum which didn't change its length with changes of temperature (which influenced its accuracy.) No mean trick, but it still had a pendulum. He then built a clock which required neither a pendulum nor lubrication, which eliminated the problems, including the viscosity changes of the lubricant, but it was made of wood and all but impossible to protect from humidity. Finally he created a metal clock of various metals whose temperature related characteristics offset each other, thus eliminating error, but it was huge--4x4 feet and weighed over 80 lbs. These efforts took him decades, and involved 3 separate clocks: H-1, H-2 and H-3. Two and three were smaller if not lighter, and the last met the requirements of the crown for accuracy, but Harrison was not satisfied.

Having commissioned the creation of a pocket watch for himself, designed by him, it occurred to him that such a device could be the answer, and using all of the now known facts might be more affordable and easier to manage, and would certainly be smaller. Eureka! Several years later he had a wonderfully accurate chronograph which was nonetheless still expensive. But it worked, and he was on his way to the prize worth millions. Unfortunately his hated adversary was the Royal Astronomer, and sabotaged both the clock and the endeavor, holding out for an astronomical answer which he and his predecessors had been working on for decades. Harrison stood alone against the vested interests of both scientists and admirals, and finally prevailed in 1759: the H-4 was a 3 pound mechanism only 5 inches in diameter and sealed completely from humidity, and it performed as well as the H-3 which had demonstrated an accuracy of but 3 seconds lost on an 81 day cruise to the Caribbean!

While it is clear that Ms. Sobel could have described, at length, the processes of both competing endeavors, she briefly reviews--bless her heart--the information while keeping the book short and interesting. In addition she is a gifted writer, reminding me of the now long gone section in the Readers Digest which was titled "toward more picturesque speech." A delightful read if you are interested in the quest for latitude; brief, but complete.

Posted by Curmudgeon at March 6, 2010 12:41 PM