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practically used. We shall not enter here upon the method of forming the tables themselves.

The following is a specimen of the way in which the logarithms of numbers are usually tabulated :

11

No.

0

1

2

3

4

5

6

7

8

9

D.

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11

16

22

27

32

38

43

49

a

6154 Thus, if the number consist of four figures only, we have simply to copy out the figures in the column headed 0, prefix a decimal point, and the proper characteristic.

Ex. Log 7991 3.9026011, log 7.995 = .9028185. When we speak of a number consisting of four figures only, wo include such numbers as :003654, 07682, &c., the number of zeros immediately following the decimal points not being counted.

Thus, log •07997 2.9029271

log .007992 = 3.9026555. When the number contains five figures, as, for instance, 79936, we look along the line containing the first four figures - viz., 7993—of the number until the eye rests upon the column headed 6, the fifth figure. We then take the first three figures of the column headed 0, and affix the four figures of the colunin headed 6 in the horizontal line of the first four figures of the number.

Thus, log 79936 = 4.9027424
log :079927 = 2.9026935.

• It will be seen from the portion of the logarithmic table above extracted, that when the first three figures of the logarithm-viz., 902–have been once printed, they are not

as

repeated, but must be understood to belong to every four. figures in each column, until they are superseded by higher figures, as 903. When, however, this change is intended to be made at any place not at the commencement of a horizontal row, the first of the four figures corresponding to the change is usually printed either in different type, or, above, with a bar over it. Thus we have above 7031, indicating that from this point we must prefix 903 instead of 902.

Thus, log 79.986 = 1.9030140,

log .0079987 = 3.9030194. 26. To find the logarithm of a number not contained in the tables.

Ex. Find the logarithm of 799-1635.

Since* the mantissa of the number 79916.35 is the same as the mantissa of the given number, and that the first five figures are contained in the tables, we may proceed as follows

(1.) Take out from the tables the mantissa corresponding to the number 79916. This is .9026337.

(2.) Take out the mantissa of the next higher number in the tables-viz., 79917. This is •9026392.

(3.) Find the difference between these mantissæ. This is called the tabular difference, being the difference of the mantissæ for a difference of unity in the numbers. We find tab. diff. •0000055, which we call D.

(4.) Then assuming that small differences in numbers are proportional to the differences of the corresponding logarithms, we find the difference for •35 •35 x .0000055 •0000019, retaining only 7 figures. This is often called d.

(5.) Now adding this value of d to the mantissa for the number 79916, we get the mantissa corresponding to the number 79916.35.

(6.) Lastly, prefix to this mantissa the proper characteristic. The whole operation may stand thus

M. of log 79916 •9026337 .............

M. of log 79917 .9026392 Tabular difference or D •0000055 * Thus log 79916.35 = log (100 x 799·1635) = 2 + log 799-1635..

(1).

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=

Hence, difference for 35 or d
= .35 .0000055 = .0000019 ...

(2). Hence, adding (1.) and (2.)—

M. of log 79916:35 •9026356.

:: log 799.1635 2.9026356. or better thus, omitting the useless ciphers

M. of log 79916 •9026337
M. of log 79917 •9026392

55
Hence, d = •35 x 55

19 .. M. of log 79916:35 •9026356, as before. In the next article we shall show how the required difference may be obtained by inspection from the tables.

27. Proportional parts.

We saw in the example just worked that the tab. diff. (omitting the useless ciphers) is 55, and if we examine the table in Art. 25, we shall find the difference between the mantissæ of any two consecutive numbers there to be 54 or 55-generally 54. The number 54 is therefore placed in a separate column at the right of the table, and headed D.

The student will understand that the tab. diff. changes from time to time, and is not always 54 or 55.

Now assuming as in (4.) of the last article, we have-. Diff. for •1 54 x .1 5 Diff. for .6 = 54 x .6 32 •2 54 x 2 - 11

7 = 54 x 57 38 •3 54 x :3 16

.8 54 x 8 43 •4 54 x 4 22

.9 54 x .9 49 •5 54 x 5 27

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We find therefore the numbers 5, 11, 16, 22, 27, 32, 38, 43, 49 placed in a horizontal row at the bottom marked P, in the columns respectively headed 1, 2, 3, 4, &c.

Hence, if we require the difference for (say) •7, we take out the number 38 from the horizontal row marked P, instead of being at the trouble to find it by actual computation.

The following example will illustrate how we proceed when we require the difference for a decimal containing more

a than one decimal figure. No explanation is needed.

the line which meets the circle, the line which meets the ciri: shall touch it.

Let any point D be taken without the circle ABC, al. from it let two straight lines, DCA, DB, be drawn, of whi DCA cuts the circle, and DB meets it; and let the rectang AD, DC be equal to the square on DB.

Then DB shall touch the circle. Draw DE CONSTRUCTION.-Draw the straight line DE, touching! touching the circle. circle ABC (III. 17);

Find F the centre (III. 1' and join FI.. FD, FE.

PROOF.— Then FED is a right ang (III. 18).

And because DE touches the circle A1 B

B and DCA cuts it, the reetangle AD, DIE is equal to the square on DE (III. 36).

But the rectangle AD, DC is equai :: the square on DB (Hyp.);

Therefore the square on DE is equiii?

the square on DB (Ax. 1); Then Therefore the straight line DE is equal to the straight li. DE = DB. DB.

And EF is equal to BF (I. Def. 15);

Therefore the two sides DE, EF are equal to the twr

sides DB, BF, each to each; And tri

And the base DF is common to the two triangles DET, angles DBF and DBF; DEF are equal in

Therefore the angle DEF is equal to the angle DBF (I.". erery te But DEF is a right angle (Const.); spect. Therefore also DBF is a right angle (Ax. 1). .:. DBF is And BF, if produced, is a diameter; and the straight li a right angle;

which is drawn at right angles to a diameter, from the ti

tremity of it, touches the circle (III. 16, Cor.); and there Therefore DB touches the circle ABC. fore DB touches Therefore, if from a point, &c. Q.E.D. the cirle

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=

log .00123 = 1.3001031 1230345

196 1230541 1230541

-4922164

2.5078867 M. of log N, M. of log 32202; d. D,

M. of log 32202.29;
log .03220229.
the number required.

ical Tables.

tables much in the same way varithms of numbers.

natural sines, cosines, &c., and ines, &c. It is with the latter gh many of our remarks apply

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tural sines and cosines of all re (Art. 11) less than unity, it

logarithms are negative. To m in a negative form, and for add 10 to their real value, and must allow for this.

The same case of logarithmic tangents, counts.

true logarithmic sine by log sin, sine by L sin.

= L sin A
= L cos A

10, &c.
in using the tables that, although
and tangent of an angle increase as

10,

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