4iQ6 Intelligence and Miscellaneous Articles. 



of two plates of 155 milliins. diameter, immersed in water, 

 amounting at first to 0*1 of a millimetre, increases, in consequence 

 of the continuous pull of 1 gramme, O01 inillim. in 1| minute, 0*1 

 inillim. in 7 minutes. From this it is intelligible how, limiting the 

 observation to a short time, one may be misled to the assumption 

 of a static equilibrium. 



The author measured, in his experiments, the times which elapsed 

 while a given initial distance, measured by a wire placed between 

 the plates, increased by a certain quantity. Between these times 

 and the other quantities which varied with the experiments the 

 following relations were found. The times are inversely propor- 

 tional to the separating force ; they are, but not exactly, inversely 

 proportional to the square of the initial distance ; for plates of 

 different sizes they are to one another as the fourth powers of the 

 radii of the plates ; for different liquids, as the times in which, 

 under equal pressure, equal volumes of the liquid flow through a 

 capillary tube. 



From this it evidently results that with this phenomenon the 

 question is a problem of hydrodynamics ; and it is now easy, at least 

 in general, to describe it. When the separating force begins to 

 act, the distance between the plates receives an infinitely small in- 

 crement. Thereby the space limited by the plates is augmented, 

 and the fluid within undergoes a dilatation, in consequence of which 

 the hydrostatic pressure becomes less. The excess of pressure of 

 the exterior fluid acts in opposition to the separating force. Never- 

 theless equilibrium does not ensue, because the diminution of the 

 hydrostatic pressure between the plates has for its result a flowing- 

 in of the exterior fluid and thereby, again, a diminution of the dif- 

 ference of the pressures. The distance of the plates can be again 

 increased by the separating force, and the same process repeats itself 

 in a continuous manner. 



The author gives also an approximative theoretical solution of 

 the problem, starting from the following consideration. The vis 

 viva acquired by the plates through the separating force is, on ac- 

 count of the great slowness of the motion, vanishingly small in com- 

 parison with the work of that force. This work must consequently 

 have its equivalent in another work ; it has it in that which is ne- 

 cessary for maintaining the flow of the fluid from the outside into 

 the space included between the plates. 



The equation deduced from this assumption gives again all the 

 different laws to which the experiments have conducted. It per- 

 mits us also to derive from the experiments the coefficients of in- 

 ternal friction for the experimental fluids. If the centimetre be 

 chosen as the unit of length, the mass of 1 gramme as the unit of 

 mass, and the second as time-unit, it follows that for water of the 

 temperature of 19° C. this coefficient = 0-0108, for air = 0*00183, 

 which values almost exactly coincide with those deduced from the 

 experiments of Poiseuille, Maxwell, and O. E. Meyer. — Kaiserliche 

 Akaclemie der Wissenschaften in Wien, Sitzung der mathematisch-na- 

 turwissenschaftlichen Classe vom 30. April 1874* 



