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magnetic compass: instrument indicating directions by a magnetic needle, see article on the compass.
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magnetic deviation: the angle between geographical north and the direction indicated by a compass needle.
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magnitudes: numbers in a scale of the relative brightnesses of the stars, 1 being the brightest.
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mariner's astrolabe: altitude-measuring instrument for finding latitude at sea, having a heavy brass or bronze ring, suspended by a shackle and ring, with a degree scale and a centrally-pivoted alidade.
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mater: part of an astrolabe, see article on the astrolabe.
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mathematical compendium: compound instrument for performing various functions in practical mathematics.
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mathematical instruments: collections of instruments of assorted kinds for performing functions in practical mathematics.
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measuring rod: an instrument used to measure distances or depths. See also gauging rod.
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measuring rods: see measuring rod.
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meridian: the plane containing the observer and both celestial poles, or containing the observer, the zenith and the pole, or, more colloquially, the north-south line at a particular position on the earth.
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meridian circle: circular scale of degrees which in use is aligned with the meridian.
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mining instruments: in this period mining instruments were principally for surveying, adapted for the special conditions of working underground in mines.
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nautical circle: generic term for a circular mathematical instrument of use in navigation, but likely to carry a range of mathematical and astronomical scales.
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nautical hemisphere: a combination of graduated arcs and circles used for navigational calculations, see article on the nautical hemisphere.
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