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Finding a Logarithm.

it, not appearing to consider that an error of a single unit in its value will give a result 10 times as great or as small as it should be.

If the characteristic thus found is negative, the negative sign is usually placed above it, that this sign may not be referred to the decimal part of the logarithm, which is always positive. But calculators are in the habit of avoiding the perplexity of a negative characteristic by subtracting its absolute value from 10, and writing the difference in its stead; and, in the use of a logarithm so written, it must not be forgotten that it exceeds the true value by 10.

Secondly. In finding the decimal part of the logarithm, the decimal point of the given number is to be wholly disregarded, and any cyphers which may precede its first significant figure on the left, or follow its last significant figure on the right are to be omitted.

When the number thus simplified is contained within the limits of the tables, which we shall regard as extending to numbers consisting of four places, the decimal part of its logarithm is found in a horizontal line with its three first figures, and in the column below its fourth figure; the second, third, and fourth figures, when wanting, being supposed to be cyphers.

When the number consists of more than four places, and is therefore, beyond the limits of the tables, point off its first four places on the left and

Finding a Logarithm.

consider them as integers, regarding the other places as decimals.

Care must be taken not to confound the decimal point thus introduced with the actual decimal point of the number, of which it is altogether independent.

Find, in the tables, the decimal logarithm corresponding to the integral part of the number thus pointed off; and also the difference between this logarithm and the one next above it, that is, the logarithm of the number which exceeds this integral part by unity; this difference is often given in the margin of the tables.

Multiply this difference by the decimal part of the number as last pointed off, and omit in the product as many places to the right as there are places in this decimal part of the number.

The product, thus reduced, being added to the decimal logarithm of the integral part of the number, is the decimal part of the required logarithm.

23. Corollary. This process for finding the decimal part of the logarithm of a number, which exceeds the limits of the tables, is founded on the following law, easily deduced from the inspection of the tables.

If several numbers are nearly equal, their differences are proportional to the differences of their logarithms.


1. Find the logarithm of 0·00325787.

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For the decimal part, the number is to be written

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3. Find the logarithm of 757-823000. Ans. 8.87956.

4. Find the logarithm of 0.00041359.

Ans. 4-61657, or 6-61657.

5. Find the logarithm of 0.12345.

Ans. 109149, or 9.09149.

6. Find the logarithm of 99998.

Ans. 4.99999.

25. Problem. To find the number corresponding to a given logarithm.

Solution. First. In finding the figures of the required number, the characteristic is to be neglected.

Number corresponding to Logarithm.

When the decimal part of the given logarithm is exactly contained in the tables, its corresponding number can be immediately found by inspection.

But when the given logarithm is not exactly contained in the tables, the number, corresponding to the logarithms of the table which is next below it, gives the four first places on the left of the required number.

One or two more places are found by annexing one or two cyphers to the difference between the given logarithm and the logarithm of the tables next below it, and dividing by the difference between the logarithm of the tables next below and that next above the given logarithm.

When tables are used in which the logarithms are given to five places, the accuracy of the corresponding numbers is never to be relied upon to more than 6 places, and rarely to more than 5 places; so that in finding the last quotient, one place is usually sufficient.

Secondly. The position of the decimal point of the required number depends altogether upon the characteristic of the given logarithm, and is easily ascertained by the rule of art. 20; cyphers being prefixed or annexed when required.


1. Find the number, whose logarithm is 8.19325.


We have for the logarithm of the tables next

below the given logarithm

19312 log. 1560.

Multiplication of Logarithms.


the diff. between given log. and log. 1560 = 13,

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gives the two additional places; so that the six places of the

required number are


and the number is, therefore,


2. Find the number, whose logarithm is 2-13511.

Ans. 136-493.

3. Find the number, whose logarithm is 1.76888.

Ans. 58-7328.

4. Find the number, whose logarithm is 0·11111.

Ans. 1.29153.

5. Find the number, whose logarithm is 2-98357.

Ans. 0.0962875.

6. Find the number, whose logarithm, when written 10 more than it should be, is 9.35846.

Ans. 0.22828.

27. Problem. To find the product of two or more factors by means of logarithms.

Solution. Find the sum of the logarithms of the factors, and the number, of which this sum is the logarithm, is, by art. 10, the required product.

When the logarithm of any of the factors is written, as in art. 22, 10 more than its true value, as many times 10 should be subtracted from the result as there are such logarithms.

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