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Book Summary: Longitude by Dava Sobel

My Personal Summary

This book is about how John Harrison figured out how to measure longitude while at sea – one of the thorniest scientific problems of the 1700’s.

Book Notes

  • The prime meridian is an imaginary line that goes through Greenwhich, England and extends from north to south, splitting the globe into Eastern and Western hemispheres.
  • It turns out that it’s easy to determine one’s latitude by measuring the length of the day or the angle of the sun relative to the horizon. However, it’s extremely difficult to measure one’s longitude because it requires you to know the time in two different locations at the same time – a task that was virtually impossible in the 1700’s even with watches because changes in temperature or barometric pressure could cause the oil inside the watches to expand or contract and speed up or slow down the watch.
  • “As more and more sailing vessels set out to conquer or explore new territories, to wage war, or to ferry gold and commodities between foreign lands, the wealth of nations floated upon the oceans. And still no shop owned a reliable means for establishing her whereabouts.”
  • Lack of understanding of longitude often meant that voyages lasted longer than they needed to, which increased the chances of crew members on ships contracting scurvy – a disease that develops from lack of fresh fruit and veggies and deficiency in vitamin C and causes connective tissue in the body to deteriorate.
  • In 1714, England’s parliament offered a reward of $12 million (in today’s currency) to anyone who could solve the longitude problem.
  • John Harrison (1693-1776), an English clockmaker, caught wind of the prize and began to work on making a clock that could measure longitude while at sea. As a child, John was obsessed with reading and learning and was even self taught when it came to clockmaking.
  • John’s first clock he made to measure longitude weighed 75 pounds and was successful on a voyage from England to Lisbon, Portugal in 1737. He nicknamed it H-1. The next iteration, H-2, weighed 65 pounds. It took 19 years for John to make the next iteration, H-3.
  • In 1759, John finishes the final version of his device, H-4, which turned out to be a watch weighing 3 pounds rather than a 60+ pound clock. John’s son, William, took H-4 on a ship from England to Jamaica and back to verify that it worked correctly.
  • Sauerkraut turned out to be the perfect food to prevent scurvy on long voyages because it’s main ingredient was cabbage which is packed with vitamin C and it can last for a very long time on a ship without spoiling.
  • The name given to watches that keep time at sea is chronometer. These became prevalent in all ships by the end of the 1700’s. Today ships just use satellites to measure longitude.

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