SLIDE (or SLIDING) RULE. The slid ing-rule is an instrument for the mechanical performance of addition and subtraction, which is converted into an instrument for the mechanical performance of multiplication and division by the use of logarithmic scales, in stead of scales of equal parts. This instru ment was invented by Oughtred, about 1630, and is very little known on the Continent.
The following are the principal kinds of sliding rule now in use. The common engi neer's rule or carpenter's rule in its best form, is a double 12-inch rule, a slide of two radii with the same scale on one side, and a scale of one radius of double length on the other, with divisors. Bevan's engineer's rule, 12 inches, has slides on both faces (which may be exchanged), and serves for squares, cubes, square roots of cubes, &c. There are scales on the backs of the slides and in the grooves, for sines, tangents, inverted numbers, com pound interest and annuities at five per cent. Henderson's double-slide rule, 12 inches, has two parallel contiguous slides, with scales of numbers fixed above and below, and solves at one operation most sets of multiplications and divisions not exceeding five operations ; at the back are tables of divisors for solids. In Wooll
gar's Pocket Calculator, 8 inches, the two slides work in either of the grooves : the backs and the grooves have scales of sines, tangents, areas of polygons, circular segments; interest, annuities, certain and for lives, at several rates of interest. An addition may be made by a metal slip, giving the solution of the same questions as the last rule. Woollgar's Pocket-Book Rule, 6 and 8 inches, has two radii, one under the other, a line for sines, and duplicate proportions at the back of the slide. At the bottom of the groove are sometimes inserted lines for finding the relations of right angled triangles, for cask gauging, and for cut tings and embankments. The Excise-officer's sliding rule, is an instrument of which the graduations are adapted to the calculation required by those officers.