
magnetic deviation: the angle between geographical north and the direction indicated by a compass needle.

magnitudes: numbers in a scale of the relative brightnesses of the stars, 1 being the brightest.

mariner's astrolabe: altitudemeasuring 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 centrallypivoted alidade.

mater: part of an astrolabe, see article on the astrolabe.

mathematical compendium: compound instrument for performing various functions in practical mathematics.

mathematical instruments: collections of instruments of assorted kinds for performing functions in practical mathematics.

measuring rod: an instrument used to measure distances or depths. See also gauging rod.

measuring rods: see measuring rod.

meridian: the plane containing the observer and both celestial poles, or containing the observer, the zenith and the pole, or, more colloquially, the northsouth line at a particular position on the earth.

meridian circle: circular scale of degrees which in use is aligned with the meridian.

mining instruments: in this period mining instruments were principally for surveying, adapted for the special conditions of working underground in mines.

nautical circle: generic term for a circular mathematical instrument of use in navigation, but likely to carry a range of mathematical and astronomical scales.

nautical hemisphere: a combination of graduated arcs and circles used for navigational calculations, see article on the nautical hemisphere.
