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More history of measurements ... extracted from:
http://home.att.net/~numericana/answer/units.htm#gallon
The US gallon, the so-called Winchester gallon, is now defined as exactly
equal to 231 cubic inches (this odd value comes from rounding up the volume
of a cylindrical measure 7 inches in diameter and 6 inches in height, which
dates back to the days of the Magna Carta). Since 1959, the inch is exactly
25.4 mm. This means that there are exactly 3785.411784 ml in a US gallon.
If the British Gallon is meant, the answer is 4546.09 ml, also an exact
value according to the 1985 British "Weights and Measures Act" (in 1963, the
British Parliament had decided to redefine all British units in metric
terms). There are about 277.42 cubic inches in this so-called Imperial
gallon.
Originally (in 1819), the Imperial gallon was meant to be the volume
occupied by 10 pounds of water at 62°F. It's intermediate in value between
the two British units it replaced in 1824, namely the corn gallon of 272¼
cubic inches (4461.378174 ml) and the ale gallon of 282 cubic inches
(4621.152048 ml). The old British wine gallon of 231 cu in survives as the
US gallon (see above)
Henry VII [Tudor] reigned from 1485 to 1509. In 1495, the Winchester bushel
was legally defined as the capacity of actual standard bushels bearing his
seal and kept at the Exchequer. In 1696, these were measured to be about
2145.6 cubic inches, under the supervision of members of the British House
of Commons who were discussing some excise duty on malt. It was then
suggested that the bushel itself be defined as a simple circular measure
roughly equivalent to this.
This was enacted in 1701 (during the reign of William III of Orange) when
the Winchester bushel was legally redefined, under the name of corn bushel,
as the capacity of "any round measure with a plain and even bottom, being
18½ inches wide throughout and 8 inches deep" (there would have been exactly
100 of these in the above container). This volume was later rounded from
2150.420171... down to exactly 2150.42 cubic inches, which is how the
so-called malt bushel has been normally defined since at least 1795. (We
couldn't determine the exact point at which the older cylindrical definition
of this bushel faded from view. Please, tell us whatever you may know.
Thanks.)
The same thing happened to the US gallon, which is a descendant of the old
Winchester wine gallon, a cylindrical measure from the days of the Magna
Carta: 7" in diameter and 6" deep, or about 230.90706 cubic inches. This
capacity was statutorily rounded to 231 cubic inches in 1707, under the
reign of Anne Stuart (it was thus once known as the Queen Anne wine
gallon).
Both Winchester units are thus tied to the inch and have, in effect, been
redefined every time the inch was. The current units of capacity are based
on the 1959 international inch, which is now forever defined in metric terms
(1" = 25.4 mm).
The US adopted the Winchester system for capacities in 1836, using the above
equivalences. The British had adopted the competing Imperial system in 1824,
on the totally different basis of an Imperial gallon then introduced as the
volume occupied by 10 lb of water at 62°F (later redefined in metric terms,
as exactly equal to 4.54609 L) and an Imperial bushel equal to 8 of these
gallons.
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