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lich may possibly be attributed to alterations in the character of the rface. The simple form (2) assumes that the wires are smooth, or else at the roughnesses are in proportion to D, so as to secure geometrical nilarity.
The completion of (5) from the theoretical point of view requires the fcroduction of v. The temperature for the experiments in which v would .ter most was about 20 C., and for this temperature
= PL =1806 x 10"7 v~ p        :00120
e generalized form of (5) is accordingly
= 1505 C.G.S.
^-.195^201" "T ~        V        FD
>plicable now to any fluid when the appropriate value of v is introduced. Dr water at 15 C., v = '0115, much less than for air.
Strouhal's observations have recently been discussed by Kriiger and Etuth*, who appear not to be acquainted with my theory. Although they > not introduce viscosity, they recognize that there is probably some cause r the observed deviations from the simplest formula (1), other than the implication arising from the circulation of the air set in motion by the volving parts of the apparatus. Undoubtedly this circulation marks a weak .ace in the method, and it is one not easy to deal with. On this account the imerical quantities in (6) may probably require some correction in order to cpress the true formula when V denotes the velocity of the wire through iherwise undisturbed fluid.
We may find confirmation of the view that viscosity enters into the .lestion, much as in (6), from some observations of Strouhal on the effect : temperature. Changes in v will tell most when VD is small, and therefore take Strouhal's table XX., where D = '0179 cm. In this there appears
= 6-48,
2=3],    F2 = 381, itroducing these into (6), we get
r with sufficient approximation
= 016 C.G.S.
^     l    -195 x 201
* "Theorie der HiebtOne," Ann. d. Physik, Vol. XMV. p. 801 (1914).hal's observations by plotting NDj V against VD. Lately I have returned to the subject, and I find that nearly all his results are fairly well represented by two terms of (3). In C.G.S. measure