1890.] 



On the Superficial Viscosity of Water. 



133 



Scale = \. 



be inserted, by which the connexion is cut off, and the circular peri- 

 phery of the well completed. The action of the apparatus depends 

 upon a stream of wind, supplied from an acoustic bellows, and 

 discharged from a glass nozzle, in a direction slightly downwards, so 

 as to strike the water surface in the tail-piece at a point a little 

 beyond the door. The effect of the wind is to carry any greasy 

 n'lm towards the far end, and thus to purify the near end of the tail- 

 piece. When the door is up, this effect influences also the water 

 surface in the well upon which the jet does not operate directly. 

 For, if the tension there be sensibly less than that of the neighbouring 

 surface in the tail-piece, an outward flow is generated, and persists as 

 long as the difference of tensions is sensible. The movements of the 

 surface are easily watched if a little sulphur be dusted over; when 

 the water in the well has been so far cleansed that but little further 

 movement is visible, the experiment may be repeated without chang- 

 ing the water by contaminating the surface with a little grease from 

 the finger or otherwise. In this way the surface may be freed from 

 an insoluble contamination any number of times, the accumulation of 

 impurity at the far end of the tail-piece not interfering with the 

 cleanness of the surface in the well. 



Another device that I have usually employed facilitates, or at any 

 rate hastens, the cleansing process. When the operation is nearly com- 

 plete, the movement of the surface becomes sluggish on account of 

 the approximate balance of tensions. At this stage the movement 



