Chapter V · Act II — Making Time
Longitude
1707–1780
A matter of life and death at sea turns precision into an affair of state.
At sea, latitude can be read in the sky; longitude cannot. To know how far east or west you have sailed, you must know the time at your home port. In 1707 four ships of the Royal Navy are wrecked on the Isles of Scilly, and between 1,400 and 2,000 sailors die. Latitude and the charts were mostly to blame, but the disaster brings the problem of finding one’s place at sea to the fore.
Time is longitude
Noon aboard · H4 shows 12:00 Portsmouth time → 1° 06′ W
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The trial voyage of H4, from Portsmouth to Jamaica by way of Madeira (1761–1762). A schematic course along great circles.
- 1Greenwich meridian
- 2Meridians: one hour every 15°
- 3Deptford’s course
- 4The ship and its meridian
The Earth turns 15° an hour. Between two meridians of this globe lies one hour.
A clock that keeps the time of the home port tells, at the ship’s noon, how far the Earth has turned since: four minutes for every degree.
On 18 November 1761 HMS Deptford sails from Portsmouth for Jamaica with John Harrison’s H4 aboard, in the care of his son William.
After 81 days and 5 hours at sea, H4 is only 5 seconds slow: 1.25 minutes of longitude, about one nautical mile.
From H1 to H4
H1
, 62 cm tall
Five years’ work for a large sea clock with two linked balances: the ship’s rolling does not disturb it. On the way back from Lisbon in 1736 it gets the ship’s position right when the master’s reckoning was some 60 miles out.
H2
1737–1739, 68.6 cm tall
Larger and sturdier, 40 kg, with a remontoire that evens out the drive to the balances. Harrison finds a flaw in them and never lets it be tried at sea.
H3
1740–1759, 62.2 cm tall
Nineteen years of work, and its two large circular balances never keep time well enough. It leaves two inventions still in use: the bimetallic strip and the caged roller bearing.
H4
, 16.5 cm tall
A large pocket watch: 12.4 cm across and 1.45 kg. Its balance beats five times a second. “Velocity was very much wanting in my three large machines”, Harrison will write.
An affair of state
The Longitude Act
Parliament offers £10,000 for a method that finds the longitude at sea to within one degree, £15,000 to within 40 minutes and £20,000 to within half a degree. The Act sets up the Board of Longitude.
Pierre Le Roy
His marine clock brings together a detent escapement, a temperature-compensated balance and an isochronous balance spring: for the historian Rupert Gould, the foundation of the modern chronometer. Fame goes to his rival, Ferdinand Berthoud.
The other clock: the Moon
Nevil Maskelyne publishes the Nautical Almanac for 1767, with the Moon’s distances from the stars worked out in advance. Against the stars the Moon moves half a degree an hour, its own width: measured with a sextant, it gives Greenwich time. It will be used until about 1850.
1772–1775
K1 and James Cook
Larcum Kendall’s copy of H4, made for £450, sails with Cook on his second voyage. Cook calls it “our trusty friend the Watch”.
Harrison’s reward
At 80, Harrison is granted £8,750 by Parliament. In all he receives £23,065 for his sea clocks, £10,000 of it in 1765 as an interim payment for H4.
Arnold and Earnshaw
They develop and patent a simpler spring detent escapement and move temperature compensation into the balance. Marine chronometers will be built this way until the electronic era. They are expensive: the Royal Navy does not issue them to all its ships until 1825.
The Line of Precision
The error a day of each chapter’s clocks, on a logarithmic scale. The line moves on with the story: each chapter draws its own stretch.
This chapter’s precision
- Graham’s regulatorc. 1715
- a few seconds a week
- Harrison’s H41761
- 5 s in 81 days
- c. 1390 BCE: Clepsydra, ≈ half an hour a day
- c. 1386: Foliot tower clock, ≈ 15 min a day
- 1656: Huygens’s pendulum, ≈ 15 s a day
- c. 1715: Graham’s regulator, a few seconds a week
- 1761: Harrison’s H4, 5 s in 81 days