The Collaborative International Dictionary of English v.0.48:
Inch \Inch\, n. [OE. inche, unche, AS. ynce, L. uncia the
twelfth part, inch, ounce. See Ounce a weight.]
1. A measure of length, the twelfth part of a foot, commonly
subdivided into halves, quarters, eights, sixteenths,
etc., as among mechanics. It was also formerly divided
into twelve parts, called lines, and originally into three
parts, called barleycorns, its length supposed to have
been determined from three grains of barley placed end to
end lengthwise. It is also sometimes called a prime ('),
composed of twelve seconds (''), as in the duodecimal
system of arithmetic.
Note: The symbol ' is the same symbol as the light accent, or
the "minutes" of an arc. The "seconds" symbol should
actually have the two strokes closer than in repeated
"minutes", but in this dictionary '' will be
interpreted as "seconds".
12 seconds ('') make 1 inch or prime. 12 inches
or primes (') make 1 foot. --B.
Note: The meter, the accepted scientific standard of length,
equals 39.37 inches; the inch is equal to 2.54
centimeters. See Metric system, and Meter.
2. A small distance or degree, whether of time or space;
hence, a critical moment; also used metaphorically of
minor concessins in bargaining; as, he won't give an inch;
give him an inch and he'll take a mile.
Beldame, I think we watched you at an inch. --Shak.
By inches, by slow degrees, gradually.
Inch of candle. See under Candle.
Inches of pressure, usually, the pressure indicated by so
many inches of a mercury column, as on a steam gauge.
Inch of water. See under Water.
Miner's inch, (Hydraulic Mining), a unit for the
measurement of water. See Inch of water, under Water.