Εικόνες σελίδας
PDF
Ηλεκτρ. έκδοση

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 :

-

[blocks in formation]

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, we include such numbers as 003654, 07682, &c., the number of zeros immediately following the decimal points not being counted.

=

2.9029271

Thus, log 07997
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 column headed 6 in the horizontal line of the first four figures of the number.

Thus, log 79936

1=

4.9027424

log '079927 2.9026935.

[ocr errors]

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

repeated, but must be understood to belong to every four figures in each column, until they are superseded by higher figures, as 903.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, as above, with a bar over it. Thus we have above 0031, 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.

[blocks in formation]

We find

(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

[blocks in formation]

........... (1).

[blocks in formation]

*Thus log 79916 35 log (100 × 799·1635) = 2 + log 799·1635..

Hence, difference for 35 or d

= 35 × 0000055 0000019..

Hence, adding (1.) and (2.)—

M. of log 79916.35

.. log 799.1635

·(2).

[blocks in formation]
[blocks in formation]

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—.

[merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][ocr errors][merged small][merged small][ocr errors][merged small]

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 than one decimal figure. No explanation is needed.

Draw ᎠᎬ touching the circle.

Then

the line which meets the circle, the line which meets the cin shall touch it.

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

Then DB shall touch the circle.

CONSTRUCTION.-Draw the straight line DE, touching ... circle ABC (III. 17);

D

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 AP and DCA cuts it, the reetangle AD, D is equal to the square on DE (III. 36). But the rectangle AD, DC is equal t the square on DB (Hyp.);

Therefore the square on DE is equal to the square on DB‍(Ax. 1);

Therefore the straight line DE is equal to the straight l:.

DE = DB. DB.

And triangles

DBF and

DEF are equal in

every respect.

..DBF is a right angle;

and therefore DB touches

the circle.

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

Therefore the two sides DE, EF are equal to the tw sides DB, BF, each to each;

And the base DF is common to the two triangles DEF
DBF;

Therefore the angle DEF is equal to the angle DBF (I. .
But DEF is a right angle (Const.);

Therefore also DBF is a right angle (Ax. 1).

And BF, if produced, is a diameter; and the straight li which is drawn at right angles to a diameter, from the extremity of it, touches the circle (III. 16, Cor.);

Therefore DB touches the circle ABC.
Therefore, if from a point, &c. Q.E.D.

[graphic]

log 00123
1230345
196

1230541

= 1.3001031

[blocks in formation]

M. of log N,

M. of log 32202;

[ocr errors]

parallel chor

M. of log 32202-29;

log 03220229.

the number required.

rical Tables.

tables much in the same way rarithms of numbers.

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

atural sines and cosines of all are (Art. 11) less than unity, it

logarithms are negative. To um in a negative form, and for add 10 to their real value, and must allow for this. The same case of logarithmic tangents, coints.

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

L sin A - 10,

[blocks in formation]

in using the tables that, although and tangent of an angle increase as

« ΠροηγούμενηΣυνέχεια »