London, 1688 · Curious Enquiries, discourse I

Noon in London

A ship can find its latitude from the sky. Its longitude needs one more thing: the time, at that moment, back home. The sun crosses the sky 15 degrees an hour, so every four minutes between your noon and London's is one degree east or west. In the seventeenth century no clock could keep London time across an ocean.

The Public Domain Review's piece on Kenelm Digby's “sympathetick powder” mentions a 1688 pamphlet that proposed wounded dogs. Here is what the pamphlet actually says, and what its ideas would have been worth.

Title page: Curious Enquiries, being six brief discourses, I. Of the Longitude ... London, printed for Randal Taylor, 1688.
Title page, 1688. Wellcome Collection copy, Internet Archive.
Page 2 of Curious Enquiries, with the passage on Sir Kenelm Digby's powder.
Page 2: the moon, the eclipses of stars, and Digby's powder.

The discourse “Of the Longitude” is three pages long and tries four ideas:

  1. An old book says a glass filled to the brim runs over at the instant of new or full moon, which would give London time twice a month. The author tried it: “when tried at several Full Moons, I never observed the Water to stir”.
  2. The moon moves against the sun and stars, so its position is a clock. But this way is “too Minute, unless we had Instruments that would observe an Altitude to a minute or less”.
  3. The moon often passes in front of a star. If an astronomer worked out those times for London in advance, a ship could compare. Near full moon, though, its glare hides all but the brightest stars.
  4. Digby's powder. Digby had told how Mr Howell started when his bloody garter went into the powdered water. If such a starting could be made to happen to an animal far away, “we might at Sea with great Ease and Pleasure know when the Sun was upon the Meridian at London”.
“Fye! says one, or other, you would not sure put a Dog to the misery of having always a Wound about him to serve you, would you? Why not as well as to keep a Dog two or three days together starving, that he may give his Master an hour or two's Pleasure the better after it.”

Would it win the prize?

Twenty-six years later the Longitude Act of 1714 offered £10,000 for longitude within one degree, £15,000 within 40 minutes of arc, and £20,000 within half a degree. Turn the dials to set how good each method is.

The moon as a clock

The starting dog

Two small corrections to the retellings

The pamphlet has no knife and no calm. Wikipedia's version has a shore-keeper dip the knife that wounded the dog, making it yelp; the Public Domain Review's has the dog's rags dipped at noon so that the dog falls quiet. In 1688 the mechanism is Digby's own: a garter in powdered water, and a start. The dog only arrives with the objection. Wikipedia also dates the pamphlet to 1687; this copy's title page says 1688.

And the Public Domain Review gives Digby's dates as 1603–55. Wikipedia has him dying on 11 June 1665.

The author's own judgement holds up best. His moon method needed instruments good “to a minute or less”, and by his own figure of 29 days for the moon's circuit, one minute of arc in the moon's position is almost exactly the half degree of longitude that would later win £20,000.

Sources: Curious Enquiries. Being six brief discourses (London: Randal Taylor, 1688), pp. 1–3, Wellcome Collection copy on the Internet Archive (b3033200x), public domain mark; quotations modernise only the long s. The Public Domain Review, “Longitude by Way of Wounded Hounds: Kenelm Digby's Sympathetick Powder (1669 edition)”. Wikipedia: “Powder of sympathy” for its version of the scheme and the year 1687, “Kenelm Digby” for 1603–1665, “Longitude Act” for the three rewards. The moon's rate, 360 degrees in 29 days, is the pamphlet's own figure; with it the moon moves about 31 minutes of arc an hour. Miles are nautical miles, 60 to a degree of longitude at the equator, fewer toward the poles. Made by Claude, 3 October 2026.

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