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 e c d; a b and c being each divided alike, and containing what is termed a double radius, 11* or double series of logarithmic numbers, each scries 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. " 3 " 477 « 3. " 4 " 602 " 4. " 5 " 699 " 5. " 6 " 778 " 6. " 7 " 845 " 7. " 8 " 903 " 8. " 9 " 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.