Tuesday, December 11, 2007

63360 inches per mile

a thread in progress:-
drawing old deed plots from deed descriptions - alt.genealogy | Google Groups

"Sometimes people complain about the British system of units, which now only the Americans use, the British having gone metric like everyone else. For example, some people don't like the fact that there are 63360 inches per mile. They say this is a silly number that's hard to remember.

These people must be nuts! The number 63360 = 27 x 32 x 5 x 11 is perfectly nice.

I can't imagine anything simpler than this. Besides, the USA reached its technological preeminence on the basis of this system, and we're not going to change now!

. . .
I quote:
Four perches measured 22 yards, a strange distance that makes sense only in the context of the traditional units used for measuring land. Like all units of land measurement, a perch, also known as a rod or a pole, originally varied according to the quality of ground: a perch of poor soil was longer than one of fertile soil, but in the course of the sixteenth century it became standardized at 16.5 feet. This inconvenient length was derived from the area of agricultural land that could be worked by one person in a day - hence the variability. The area was reckoned to be 2 perches by 2 perches (33 feet by 33 feet). Thus a daywork amounted to 4 square perches. Conveniently, there were 40 dayworks in an acre, the area that could be worked by a team of oxen in a day, and 640 acres in a square mile. It was significant that all of them were multiples of 4, a number that made it simpler to calculate the area of a four-sided field.

Gunter divided the chain into 100 links, marked off into groups of 10 by brass rings. On the face of it, the dimensions make no sense: each link is a fraction under 8 inches long; 10 links make slightly less than 6 feet, 8 inches; and the full length is 66 feet. In fact, he had made a brilliant synthesis of two otherwise incompatible systems, the traditional English land measurements, based on the number 4, and the newly introduced system of decimals based on the number 10.

The classic text Boundary Control and Legal Principles by Brown, et al., notes that in the 16th century, the rod was established as the length of the left feet of the first 16 men out of church one Sunday morning. Brown cites "The Amazing Story of Measurements." This same story is often repeated in surveying texts and publications. Indeed, it is a part of the folklore of the surveying profession.

While the story that the rod was originally meant to represent 16 human feet is probably true, the reality is that the rod had been in common use many centuries earlier than Brown suggested. As early as 1270, Edward I of England issued a Royal ordinance titled Assize of Weights and Measures, which at this date defined into law the length of a perch. This indicates that the perch was in use prior to this time.

Usually, a larger measurement is defined in terms of being an equal number of a smaller measurement, much in the sense that one pound is considered to be 16 ounces. And yet as early as 1270, the perch is not considered as 16 feet but rather specifically defined as 16 1/2 feet.



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