Set _r_ on C to 57·3 on D, and over any number of degrees _n_ on D read the (approximate) length of the arc on C. EX.—_r_ = 24; _n_ = 30; find _l_. Set 24 on C to 57·3 on D, and over 30 on D read 12·56 = _l_ on C. Given the diameter _d_ of a circle in _inches_, to find the circumference _c_ in _feet_. Set 191 on C to 50 on D, and under any diameter in inches on C read circumference _c_, in feet on D. EX.—Find the circumference in feet of a pulley 17 in. in diameter. Set 191 on C to 50 on D, and under 17 on C read 4·45 ft. on D. Given the diameter of a circle, to find its area. Set 0·7854 on B to 10 (centre index) on A and over any diameter on D read area on B. When the rule has a special graduation line = 0·7854, on the right-hand scale of B, set this line to the R.H. index of A and read off as above. If only π is marked, set this special graduation on B to 4 on A. On the C and D scales of some rules a gauge point marked _c_ will be found indicating √((4)/(π)) = 1·1286. In this case, therefore, set 1 on C to gauge point _c_ on D, and read area on A as above. If the gauge point _c_′ is used, divide the result by 10. Or set _c_ on C, to diameter on D, and over index of B read area on A. Cursors are supplied, having _two_ lines ruled on the glass, the interval between them being equal to (4)/(π) = 1·273 on the A scale. In this case, if the right hand of the two cursor lines be set to the diameter on D, the _area_ will be read on A under the _left_-hand cursor line. For diameters less than 1·11 it is necessary to set the middle index of B to the L.H. index of A, reading the areas on the L.H. B scale. The confusion which in general work is sometimes caused by the use of two cursor lines might be obviated by making the left-hand line in two short lengths, each only just covering the scales. Given diameter of circle _d_ in _inches_, to find area _a_ in square _feet_. Set 6 on B to 11 on A, and over diameter in inches on D read area in square feet on B. To find the surface in square feet of boiler flues, condenser tubes, heating pipes, etc., having given the diameter in inches and length in feet. Find the circumference in feet as above and multiply by the length in feet. EX.—Find the heating surface afforded by 160 locomotive boiler tubes 1¾in. in diameter and 12 ft. long. Set 191 on C to 50 on D; bring cursor 1·75 on C, L.H. index of C to cursor; cursor to 12 on C; 1 on C to cursor; and under 160 on C read 880 sq. ft. of heating surface on D. If the dimensions are in the same denomination and the rule has a gauge point M at 31·83 (= (100)/(π)), set this mark on B to diameter of cylinder on A, and read cylindrical surface on A over length on B. To find the side _s_ of a square, equal in area to a given rectangle of length _l_ and breadth _b_. Set R.H. or L.H. index of B to _l_ on A, and under _b_ on B read _s_ on D.