Electric light (1 set) 299850 Set micrometer counting oscillations (2) 299840 Readings taken by Lieutenant Nazro (3) 299830 Readings taken by Mr. Clason (5) 299860 Mirror inverted (8) 299840 Speed of rotation, 192 (7) 299990 Speed of rotation, 128 (1) 299800 Speed of rotation, 96 (1) 299810 Speed of rotation, 64 (1) 299870 Radius, 28.5 feet (54) 299870 Radius, 33.3 feet (46) 299830 Highest temperature, 90° Fahr. (5) 299910 Mean of lowest temperatures, 60° Fahr. (7) 299800 Image, good (46) 299860 Image, fair (39) 299860 Image, poor (15) 299810 Frame, inclined (5) 299960 Greatest value 300070 Least value 299650 Mean value 299852 Average difference from mean 60 Value found for π 3.26 Probable error ± 5 Discussion of Errors. The value of V depends on three quantities D, n, and φ. These will now be considered in detail. The Distance. The distance between the two mirrors may be in error, either by an erroneous determination of the length of the steel tape used, or by a mistake in the measurement of the distance by the tape. The first may be caused by an error in the copy of the standard yard, or in the comparison between the standard and the tape. An error in this copy, of .00036 inch, which, for such a copy, would be considered large, would produce an error of only .00001 in the final result. Supposing that the bisections of the divisions are correct to .0005 inch, which is a liberal estimate, the error caused by supposing the error in each yard to be in the same direction would be only .000014; or the total error of the tape, if both errors were in the same direction, would be 000024 of the whole length. The calculated probable error of the five measurements of the distance was ±.000015; hence the total error due to D would be at most .00004. The tape has been sent to Professor Rogers, of Cambridge, for comparison, to confirm the result. The Speed of Rotation. This quantity depends on three conditions. It is affected, first, by an error in the rate of the standard; second, by an error in the count of the sound beats between the forks; and third, by a false estimate of the moment when the image of the revolving mirror is at rest, at which moment the deflection is measured.