HERMANN VON HELMHOLTZ 



of an electrified plate of finite size parallel to an infinite 

 plane surface connected with the earth. 



It was important for hydrodynamical theories to 

 determine with exactitude the internal friction of 

 fluids. Two important researches were undertaken. 

 In one, along with G. von Piotrowski, Helmholtz 

 observed the friction that occurs at the borders of 

 the fluid where it touches the surrounding medium. 

 A globe of metal, having a polished and gilded 

 interior, was filled with various fluids and swung 

 round a perpendicular axis, while the delay in the 

 vibrations or movements at the border was observed 

 and measured by means of a mirror reflecting a beam 

 of light into a telescope. This method was similar 

 to that previously employed by Bessel, whose obser- 

 vations and mathematical deductions are fully dis- 

 cussed. In long thin tubes, as was shown by 

 Poiseuille, the upper layer of fluid adheres firmly to 

 the walls of the tube, but this was shown not 

 to be the case with the polished globe when filled 

 with water, though it was true as regarded alcohol 

 and ether. The other research dealing with ques- 

 tions regarding friction was mathematical, and com- 

 pleted the theory of stationary or static streams in 

 fluids. 



The theorem as to the geometrically similar move- 

 ments of fluid bodies, issued in 1873, made it possible 

 to draw some practical conclusions regarding the 

 steering of balloons, with which the behaviour of a 



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