The Scales.

The
upper scale line of the rule consists of two exactly Similar
scales, placed consecutively. The lower scale is like the
upper one, but is twice the length.

The
front of the slide is divided the same as the rule, so that
when the 1 of the slide is placed against the 1 of the rule,
all the other divisions coincide. The back of the slide is
marked with three scales, one being a scale of sines, indicated
by the letter S; the second, a scale of tangents, marked
T; and the third, a scale of equal parts, for use when
the logarithm of a number is required.


The Divisions.

Each of
the upper scales is divided into 10 primary parts, numbered
from 1 to 10, which are spaced according to the logarithm of
the numbers, and each of these is subdivided into 10 parts.
Between 1 and 2, these last divisions are each divided into
5 parts, and between 2 and 5 into 2 parts.

The lower
scale, which is used to a much greater



extent than any other scale on the rule, is like the upper one,
first divided into 10 parts, and each of these divided into 10
parts. Between 1 and 2, the subdivision of the main space
are each divided into 10 parts; between 2 and 4, they are
divided into 5 parts; and for the remainder of the scale,
into 2 parts only.

The 1 mark
on the left hand of the scale may represent 1, 10, 100, 0.1, 0.01,
0.001, etc., the positions of the decimal point being of no
importance; but once the value has been—for particular
calculation—assigned to it, the ratio of the values must be
maintained throughout the scale. Thus, if it is taken to
represent 10, the main divisions must be read as 20, 30, 40, etc.
The figured subdivisions of the main space 12 will in this case
be read as 11, 12, 13, etc., and the small spaced between 1 and 2
will represent 0.1 each, being onehundredth of the main division,
so that the second mark to the right of the 6 figured between
1 and 2 will read as 16.2.

Between
2 and 4 the small spaces will represent 0.2 each, being
onefiftieth of the main division and those between.

