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Plate
4 from Americae Retectio |
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Instruments
of Navigation |
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Deceptively like a common sundial, William Borough's instrument to determine magnetic variation pushed the envelope of scientific understanding in 1581. But more than that, it encapsulated a thorny scientific problem within a very simple instrument—so simple that a sailor could use it. Since the magnetic poles do not lie in the same place on the earth's globe as the true north and south poles, a compass needle will usually not point directly at the geographic pole. The deviation depends on where the compass is in relation to the magnetic pole as well as local geomagnetic conditions. Knowing this "magnetic variation" was of crucial importance to navigators trying to avoid being lost at sea. It was of particular concern to those plying the North Atlantic. Near the equator, variation might only be a degree or so, but in northern waters, it can vary up to 90°. Near Greenland, navigators could conceivably be sailing west when they thought they were sailing north. Borough therefore
put into a simple instrument all the tools that the navigator needed
to find and map variation at any point on the globe. The instrument worked as follows: Placing the instrument level and plumb on a stool and aligning the shadow from the cord on the N-S line of the compass dial, the navigator noted the position of the needle at two times in the day when the sun was at equal elevations above the horizon—that is, at equal times on either side of local noon. Half the difference in the absolute value of these two needle positions gave the local magnetic variation since magnetic north is not in quite the same direction as true north (the north magnetic pole lies at about 77° N lat., 45° W long.). So, for example, at 8 AM the needle might point 48°E and at 4pm point 45°W; the local magnetic variation would then be 1.5°E ([48°-45°]/2) at that point on the globe. The next time any ship came to the area, then, it would be able to compensate for that 1.5° and sail towards its destination reliably. One limitation of this instrument, however, was that to determine the most accurate declinations, the readings ought to have been taken as far apart in the day as possible, but that meant the mariner had to stay put for most or all of that day, not only a difficult task on the ocean, but also quite inefficient in terms of reaching one's destination. Ultimately, though, this instrument and many more like it defined the mathematical arts for late-sixteenth-century Englishmen and the world they sought to create. Steven A.
Walton
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