In order to avoid the nuisance of fractions, Napier took the length of AB as〖10〗^7,for the best tables of sines available to him extended to even places. From Napier's definition, and through the use of knowledge not available to Napier, it develops that*
Nap log y = 〖10〗^7, log 1/e (y/〖10〗^7),
go that the frequently made statement that Napierian logarithms are natural logarithms is actually without basis. One observes that the Napierian logarithm decreases as the number increases, contrary to what happcns with natural logarithms.
*The result is easily shown with the aid of a little calculus. Thus we have AC = 〖10〗^7- y_1 whence
Velocity of C = -dy/dt = y.
That is, dy/y = -dt, or integrating, ln y = -t + C. Evaluating the constant of integration by substituting t = 0, we find that C = ln 〖10〗^7, whence
ln y = -t + ln〖10〗^7
Now
Velocity of F = dx/dt = 〖10〗^7,
so that x = 〖10〗^7t. Therefore
Nap log y = x = 〖10〗^7t =〖10〗^7 (ln 〖10〗^7- ln y)
= 〖10〗^7ln (〖10〗^7/y) = 〖10〗^7log1/4 (y/〖10〗^7).
It further develops that, over a succession of equal periods of time, y decreases in geometric progression while x increases in arithmetic progression. Thus we have the fundamental principle of a system of logarithms, the association of a geometric and an arithmetic progression. It now follows, for example, that if a/b = c/d, then
Nap log a - Nap log b = Nap log c - Nap log d,