The slide rule is an instrument by which the greater portion of operations in arithmetic and mensuration may be advantageously performed, provided the lines of division and gauge-points be made properly correct, and their several values familiarly understood.

The lines of division are distinguished by the letters a b c D; A B and c being each divided alike, and containing what is termed a double radius, or double series of logarithmic numbers, each series being supposed to be divided into 1000 equal parts, and distributed along the radius in the following manner:

From 1 to

2

contains

301

of those parts,

being

the log. of

2.

"

8

477

"

"

"

8.

"

4

602

"

"

■ 4

4.

"

5

699

"

"

"

5.

"

8

778

"

"

"

6.

"

7

845

"

"

"

7.

"

8

903

"

"

"

8.

"

0

954

"

"

"

9.

1000 being the whole number.

The line d on the improved rules consists of only a single radius; and although of larger radius, the logarithmic series is the same, and disposed of along the line in a similar proportion, forming exactly a line of square roots to the numbers on the lines B c.