The observations were conducted as follows:—Around the cast-iron trough were sixteen equidistant marks. The apparatus was revolved very slowly (one turn in six minutes) and after a few minutes the cross wire of the micrometer was set on the clearest of the interference fringes at the instant of passing one of the marks. The motion was so slow that this could be done readily and accurately. The reading of the screw-head on the micrometer was noted, and a very slight and gradual impulse was given to keep up the motion of the stone; on passing the second mark, the same process was repeated, and this was continued till the apparatus had completed six revolutions. It was found that by keeping the apparatus in slow uniform motion, the results were much more uniform and consistent than when the stone was brought to rest for every observation; for the effects of strains could be noted for at least half a minute after the stone came to rest, and during this time effects of change of temperature came into action.
The following tables give the means of the six readings; the first, for observations made near noon, the second, those near six o'clock in the evening. The readings are divisions of the screw-heads. The width of the fringes varied from
, the mean value being near
, so that one division means
. The rotation in the observations at noon was contrary to, and in the evening observations, with, that of the hands of a watch.
NOON OBSERVATIONS.
| 16. | 1. | 2. | 3. | 4. | 5. | 6. | 7. | 8. | |
| July 8 | 44.7 | 44.0 | 43.5 | 39.7 | 35.2 | 34.7 | 34.3 | 32.5 | 28.2 |
| July 9 | 57.4 | 57.3 | 58.2 | 59.2 | 58.7 | 60.2 | 60.8 | 62.0 | 61.5 |
| July 11 | 27.3 | 23.5 | 22.0 | 19.3 | 19.2 | 19.3 | 18.7 | 18.8 | 16.2 |
| Mean | 43.1 | 41.6 | 41.2 | 39.4 | 37.7 | 38.1 | 37.9 | 37.8 | 35.3 |
| Mean in w. l. | .862 | .832 | .824 | .788 | .754 | .762 | .758 | .756 | .706 |
| .706 | .692 | .686 | .688 | .688 | .678 | .672 | .628 | .616 | |
| Final mean | .784 | .762 | .755 | .738 | .721 | .720 | .715 | .692 | .661 |