Log·a·rithm n.  Math. One of a class of auxiliary numbers, devised by John Napier, of Merchiston, Scotland (1550-1617), to abridge arithmetical calculations, by the use of addition and subtraction in place of multiplication and division.
 Note: The relation of logarithms to common numbers is that of numbers in an arithmetical series to corresponding numbers in a geometrical series, so that sums and differences of the former indicate respectively products and quotients of the latter; thus,
 0    1    2     3      4       Indices or logarithms
 1   10   100  1000  10,000     Numbers in geometrical progression
 Hence, the logarithm of any given number is the exponent of a power to which another given invariable number, called the base, must be raised in order to produce that given number. Thus, let 10 be the base, then 2 is the logarithm of 100, because 10² = 100, and 3 is the logarithm of 1,000, because 10³ = 1,000.
 Arithmetical complement of a logarithm, the difference between a logarithm and the number ten.
 Binary logarithms. See under Binary.
 Common logarithms, or Brigg's logarithms, logarithms of which the base is 10; -- so called from Henry Briggs, who invented them.
 Gauss's logarithms, tables of logarithms constructed for facilitating the operation of finding the logarithm of the sum of difference of two quantities from the logarithms of the quantities, one entry of those tables and two additions or subtractions answering the purpose of three entries of the common tables and one addition or subtraction.  They were suggested by the celebrated German mathematician Karl Friedrich Gauss (died in 1855), and are of great service in many astronomical computations.
 Hyperbolic logarithm or Napierian logarithm or Natural logarithm, a logarithm (devised by John Speidell, 1619) of which the base is e (2.718281828459045...); -- so called from Napier, the inventor of logarithms.
 Logistic logarithms or  Proportional logarithms, See under Logistic.

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 Lo·gis·tic Lo·gis·tic·al a.
 1. Logical. [Obs.]
 2. Math. Sexagesimal, or made on the scale of 60; as, logistic, or sexagesimal, arithmetic.
 Logistic logarithms, or Proportional logarithms, certain logarithmic numbers used to shorten the calculation of the fourth term of a proportion of which one of the terms is a given constant quantity, commonly one hour, while the other terms are expressed in minutes and seconds; -- not now used.

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 Pro·por·tion·al a.
 1. Having a due proportion, or comparative relation; being in suitable proportion or degree; as, the parts of an edifice are proportional.
 2. Relating to, or securing, proportion.
 3. Math. Constituting a proportion; having the same, or a constant, ratio; as, proportional quantities; momentum is proportional to quantity of matter.
 Proportional logarithms, logistic logarithms. See under Logistic.
 Proportional scale, a scale on which are marked parts proportional to the logarithms of the natural numbers; a logarithmic scale.
 Proportional scales, compasses, dividers, etc. Draughting, instruments used in making copies of drawings, or drawings of objects, on an enlarged or reduced scale.