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th the minimum of intermission. I will take an example relating to the ain rod, whose diameter (D) is 8| mm.,r = 60/106 sec., beats of metronome i in SO sec. The speed is recorded by the number of beats corresponding the passage of two spokes, and the vibration of the pendulum (after the pse of a sufficient time) is described as small, fair, good, and so on. Thus on 30. 21, 1914:
2 spokes to 4 beats gave fair vibration,
....... 5 ....... good ......
....... 6 ..... rather more . . .
....... 7 ....... good ......
....... 8 ....... fair ......
mi which we may conclude that the maximum effect corresponds to 6 beats, to a time (T) of revolution of the turn-table equal to 2 x 6 x 30/62 sec. le distance (r) of the rod from the axis of rotation was 116 mm., and the eed of the water, supposed to move with the basin, is ZirrjT. The result the observations may intelligibly be expressed by the ratio of the distance ivelled by the water during one complete vibration of the pendulum to the rnieter of the latter, viz.
T . 2?rr/y_ 2-7T x 116 x 62
D ~~ 8-5 x 6 x 100 ~
mcordant numbers were obtained on other occasions. In the above calculation the speed of the water is taken as if it were
connected with the basin, and must be an over estimate. When the ndulum is away, the water may be observed to move as a solid body after e rotation has been continued for two or three minutes. For this purpose e otherwise clean surface may be lightly dusted over with sulphur. But len the pendulum is immersed, the rotation is evidently hindered, and that t merely in the neighbourhood of the pendulum itself. The difficulty ence arising has already been referred to in connexion with Strouhal's periments and it cannot easily be met in its entirety. It may be mitigated increasing r, or by diminishing D. The latter remedy is easily applied up a certain point, and I have experimented with rods 5 mm. and 3£ mm. in imeter. With a 2 mm. rod no vibration could be observed. The final suits were thus tabulated :
Diameter ... 8'5 mm. 5-0 mm. 3'5 mm.
Ratio ... 8-35 ' 7'5 7'8
im which it would appear that the disturbance is not very serious. The ference between the ratios for the 5'0 mm. and 3'5 mm. rods is hardly out-le the limits of error; and the prospect of reducing the ratio much below 7 urned remote.
The instinct of an experimenter is to try to get rid of a disturbance, even ough only partially; but it is often equally instructive to increase it. The R. vi. 21