For the _first_ 10 turns the _average_ value of D′ is 38.155 20 turns 38.153 30 turns 38.152 40 turns 38.151 50 turns 38.149 60 turns 38.148 70 turns 38.146 80 turns 38.144 90 turns 38.142 100 turns 38.140 110 turns 38.138 120 turns 38.135 130 turns 38.132 140 turns 38.130 On comparing the scale with the standard meter, the temperature being 16°.5 C., 140 divisions were found to = 139.462mm. This multiplied by (1 + .0000188 × 16.5) = 139.505mm. One hundred and forty divisions were found to be equal to 140.022 turns of the screw, whence 140 turns of the screw = 139.483mm, or 1 turn of the screw = 0.996305mm. This is the _average_ value of one turn in 140. But the average value of D, for 140 turns is, from the preceding table, 38.130. Therefore, the true value of D, is 38.130 × .996305mm, and the average value of one turn for 10, 20, 30, etc., turns, is found by dividing 38.130 × .996305 by the values of D;, given in the table. This gives the value of a turn-- mm. For the first 10 turns 0.99570 20 turns 0.99570 30 turns 0.99573 40 turns 0.99577 50 turns 0.99580 60 turns 0.99583 70 turns 0.99589 80 turns 0.99596 90 turns 0.99601 100 turns 0.99606 110 turns 0.99612 120 turns 0.99618 130 turns 0.99625 140 turns 0.99630 NOTE.--The micrometer has been sent to Professor Mayer, of Hoboken, to test the screw again, and to find its value. The steel tape has been sent to Professor Rogers, of Cambridge, to find its length again. (See page 145.) Measurement of the Distance between the Mirrors. Square lead weights were placed along the line, and measurements taken from the forward side of one to forward side of the next. The tape rested on the ground (which was very nearly level), and was stretched by a constant force of 10 pounds. The correction for length of the tape (100.006) was +0.12 of a foot. To correct for the stretch of the tape, the latter was stretched with a force of 15 pounds, and the stretch at intervals of 20 feet measured by a millimeter scale.