The standard yard was brought under the microscopes of the comparator; the cross-hair of the unmarked microscope was made to bisect the division marked o, and the cross-hair of the microscope, marked I, was made to bisect the division marked 36. The reading of microscope I was taken, and the other microscope was not touched during the experiment. The standard was then removed and the steel tape brought under the microscopes and moved along till the division marked 0.1 (feet) was bisected by the cross-hair of the unmarked microscope. The screw of microscope I was then turned till its cross-hair bisected the division marked 3.1 (feet), and the reading of the screw taken. The difference between the original reading and that of each measurement was noted, care being taken to regard the direction in which the screw was turned, and this gave the difference in length between the standard and each succesive portion of the steel tape in terms of turns of the micrometer-screw. To find the value of one turn, the cross-hair was moved over a millimeter scale, and the following were the values obtained: Turns of screw of microscope I in 1mm-- 7.68 7.73 7.60 7.67 7.68 7.62 7.65 7.57 7.72 7.70 7.64 7.69 7.65 7.59 7.63 7.64 7.55 7.65 7.61 7.63 Mean =7.65 Hence one turn = 0.1307mm. or = 0.0051 inch. The length of the steel tape from 0.1 to 99.1 was found to be greater than 33 yards, by 7.4 turns =.96mm +.003 feet. Correction for temperature +.003 feet. Length 100.000 feet. -------------- Corrected length 100.006 feet. Determination of the Value of Micrometer. Two pairs of lines were scratched on one slide of the slit, about 38mm apart, i.e., from the center of first pair to center of second pair. This distance was measured at intervals of 1mm through the whole length of the screw, by bisecting the interval between each two pairs by the vertical silk fiber at the end of the eye-piece. With these values a curve was constructed which gave the following values for this distance, which we shall call D′: Turns of screw. At 0 of scale D′ =38.155 10 of scale D′ 38.155 20 of scale D′ 38.150 30 of scale D′ 38 150 40 of scale D′ 38.145 50 of scale D′ 38.140 60 of scale D′ 38.140 70 of scale D′ 38.130 80 of scale D′ 38.130 90 of scale D′ 38.125 100 of scale D′ 38.120 110 of scale D′ 38.110 120 of scale D′ 38.105 130 of scale D′ 38.100 140 of scale D′ 38.100 Changing the form of this table, we find that,--