A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Don Seltzer
Date: 2018 Feb 11, 23:35 +0000
'Teacheth, by three numbers given, to find out a fourth,
in such proportion to the third as the second is to the first.
RULE. - First state the question; that is place the
numbers in such order that the first and third be of one
kind, and the second the same as the number required;...
Multiply the second and third numbers together, and
divide product by the first, the quotient will be the
answer to the question...'
In simple terms, how to solve a ratio problem.
Today we would most likely express it as a very simple algebraic
equation. In earlier times it was taught as part of basic arithmetic
as a rote procedure; put the known values in a specific order,
mulitply, divide, and you have the answer.
I'm reading the "History of Nautical Astronomy", Mr Cotter describes the state of (apparently insufficient) mathematical literacy of seamen of 15th century like that:
...importance in introducing to the English seamen ... the elements of mathematics beyond the golden rule of three.
What is that?
Recently I had some hard time guessing what the "butterfly multiplication method" is. We, in Russia, were never tought anything like that.