1691 - A Bell & a Compass

Edmund Halley, an astronomer and polymath aged 35, already famed for quite a few achievements in astronomy and meteorology, presents two inventions at the Royal Society : (1) a diving bell ; (2) a damped compass.

Diving Bell

Halley has been thinking about a diving bell meant to working underwater for a while. Two years ago, he produced a paper describing a bell moving on its four wheels on sea bottom, fitted with an air intake so as to maintain water off the device despite pressure increasing with depth.

Robert Hooke, 56 by now, had already produced a similar paper twenty-five years before. He deigned to show his discontent. A great scholar, the inventor of the law of elasticity, and perhaps of the inverse square law for gravitation before Isaac Newton, tends to be somewhat fussy.

Halley had heard of William Phips' bell, which enabled his inventor to fish 30 tons of silver from a wreck four years ago. This piece of news prompted his imagination to improve what he considered as the prototype of a diving bell. He likely ignored that a communication to the Journal des Sçavans in 1678 related a testimony by an eye witness of some successful usage of a diving bell in 1659 by Spanish mariners to recover silver coins from a wreck off Cadaqués. No detail is known of those two inventions which already worked.

This year, Halley encountered the opportunity of implementing his ideas thanks to a shipwreck. The Guynie from the Royal African company foundered somewhere between Portsmouth and Brighton with its cargo of elephant tusks, redwood and bees wax. Halley built the diving bell for the purpose of recovering cargo. Of course, ivory is more appealing than wax. Let us depict his work - he calls a tub what is named a bell - and his trials, including his own participation :

Edmund Halley, reports to the Royal Society describing his device, the first trial and his own test

Halley paid with his very self : he suffered from a damage to one of his ears in this test. We do not know if Halley's team has recovered anything from the wreck yet. The Guynie wreck site is rocky from records, which is challenging to anchoring the bell.

Compass

As we all know, dead reckoning navigation requires course and speed to draw the ship route. They are read from a compass and a log (or loch). We already examine the log question in a chronicle of 1669. Let us look into the compass one. Halley's compass is filled of viscous liquid so as to damp the movements of the needle at sea. The stake is to improve its steadiness, hence, hopefully, its resolution.

The current compass is composed of a magnet needle rotating around its axis to point to magnetic North.

¤ An old belief assigns magnetic attraction to Stella Maris, also known as Polaris, or Alpha Ursae Minoris for the astronomers, instead of being of terrestrial origin.

The compass rose rotates with the needle and displays the course – its appearance only since there are corrections to work out – to the cursor in line with the ship axis.

On ocean-going vessels, this compass rose is currently a 32-point one. The angle between two points is therefore 11¼ degrees, but the learned mariners seek down to the half-point, weather permitting, to determine their course with an accuracy of 3 or 4 degrees at best. Many settle for twice this precision. This results in an uncertainty of position which amounts to 5 to 10% of the distance – 150 to 300 miles if crossing the Atlantic ocean without having the possibility of re-locating through a celestial position from time to time.

¤ For the record, here are the 32 points – naming them correctly in the right order is an exercise for the helmsmen known as ‘boxing the compass’ : North, North-by-East, North-Northeast, Northeast-by-North, Northeast, Northeast-by-East, East-Northeast, East-by-North, East, East-by-South, East-Southeast, Southeast-by-North, Southeast, Southeast-by-South, South-Southeast, South-by-East, South, South-by-West, South-Southwest, Southwest-by-South, Southwest, Southwest-by-North, West-Southwest, West-by-South, West, West-by-North, West-Northwest, Northwest-by-West, Northwest, Northwest-by-North, North-Northwest, North-by-West.

The purist will add the half-points, e.g. East-by-South half-South, to the thirty-two points.

Moreover, to transpose an order to the helmsman in a course drawn on the map, and vice versa, one has also to take into account :

(a) the drift between ship heading and its real course, which varies with the forces of the waves and of the wind, and which is left to mariner’s estimate,

(b) the local declination between magnetic North and true North, which chiefly varies with longitude,

(c) the compass bias due to magnetic materials in its surroundings, which must be nullified or, alternatively, metered from time to time,

(d) mechanical shocks and vibrations from waves and wind which make the needle swing around a mean indication, which has to be assessed.

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LINK WITH A PREVIOUS CHRONICLE

1687 - A Successful Treasure Hunt

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IN RETROSPECT FROM TODAY

NOTE - On the aftermath.

There is no public record of a success in ivory fishing. It has been said that the diving bell was not fully workable under real conditions. There is no record of usage of his diving suit either.

Anyway Halley will not be finished with maritime questions. He will especially work on magnetic variations so as to improve navigation.

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SUGGESTED BIBLIOGRAPHY

Halley's Clerk (pseudonym, of course) - Halley's Blog - Internet, 2014-16

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CREDIT

Drawing of Halley's Diving Bell - 19th century - © Science Museum