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322 JiOLIAN .TONES [394
observations of Dec. 21 were made with this object in view; besides those already given they .included others in which the disturbance due to the vibrating pendulum was augmented by the addition of a similar rod (8£ mm.) immersed to the same depth and situated symmetrically on the same diameter of the basin. The anomalous effect would thus be doubled. The record was as follows :
2 spokes to 3 beats gave little or no vibration,
.......6 ....... less . ......
.......• 1 ...... little or no ......
As the result of this and another day's similar observations it was concluded that the 5 beats with additional obstruction corresponded with 6 beats without it. An approximate correction for the disturbance due to improper action of the pendulum may thus be arrived at by decreasing the calculated ratio in the proportion of 6 : 5; thus
ft (8-35) = 7-0
is the ratio to be expected in a uniform stream. It would seem that this cannot be far from the mark, as representing the travel at a distance from the pendulum in an otherwise uniform stream during the time of one complete vibration of the latter. Since the correction for the other diameters will be decidedly less, the above number may be considered to apply to all three diameters experimented on.
In order to compare with results obtained from air, we must know the value of vjVD. For water vat 15° C. v = p= '0115 C.G.S.; and for the 8'5 mm. pendulum v/VD = '0011. Thus from (6) it appears that ND/V should have nearly the full value, say 190. The reciprocal of this, or 5*3, should agree with the ratio found above as 7'0; and the discrepancy is larger than it should be.
An experiment to try whether a change of viscosity had appreciable influence may be briefly mentioned. Observations were made upon water heated to about 60" C. and at 12° C. ISTo difference of behaviour was detected. At 60° C. fi = -0049, and at 12° C. /* = '0124.
I have described the simple pendulum apparatus in some detail, as apart from any question of measurements it demonstrates easily the general principle that the vibrations are transverse to the stream, and when in good action it exhibits very well the double row of vortices as witnessed by dimples upon the surface of the water.
The discrepancy found between the number from water (7'0) and that derived from Strouhal's experiments on air (5'3) raises the question whether 21