mm. At 100 feet the stretch was 8.0 80 feet the stretch was 5.0 60 feet the stretch was 5.0 40 feet the stretch was 3.5 20 feet the stretch was 1.5 --- --- 300 23.00 Weighted mean = 7.7 mm. For 10 pounds, stretch = 5.1 mm. = 0.0167 feet. Correction for whole distance = +0.33 feet. The following are the values obtained from five separate measurements of the distance between the caps of the piers supporting the revolving mirror and the distant reflector; allowance made in each case for effect of temperature: 1985.13 feet. 1985.17 feet. 1984.93 feet. 1985.09 feet. 1985.09 feet. ------- Mean = 1985.082 feet. +.70. Cap of pier to revolving mirror. +.33. Correction for stretch of tape. +.12. Correction for length of tape. -------- 1986.23. True distance between mirrors. Rate of Standard Ut₃ Fork. The rate of the standard Ut₃ fork was found at the Naval Academy, but as so much depended on its accuracy, another series of determinations of its rate was made, together with Professor Mayer, at the Hoboken Institute of Technology. _Set of determinations made at Naval Academy._ The fork was armed with a tip of copper foil, which was lost during the experiments and replaced by one of platinum having the same weight, 4.6 mgr. The fork, on its resonator, was placed horizontally, the platinum tip just touching the lampblacked cylinder of a Schultze chronoscope. The time was given either by a sidereal break-circuit chronometer or by the break-circuit pendulum of a mean-time clock. In the former case the break-circuit worked a relay which interrupted the current from three Grove cells. The spark from the secondary coil of an inductorium was delivered from a wire near the tip of the fork. Frequently two sparks near together were given, in which case the first alone was used. The rate of the chronometer, the record of which was kept at the Observatory, was very regular, and was found by observations of transits of stars during the week to be +1.3 seconds per day, which is the same as the recorded rate. Specimen of a Determination of Rate of Ut₃ Fork. Temp.=27° C. Column 1 gives the number of the spark or the number of the second. Column 2 gives the number of sinuosities or vibrations at the corresponding second. Column 3 gives the difference between 1 and 11, 2 and 12, 3 and 13, etc.