1890.] 



on Foam. 



95 



Fig. 4. 



needle requires (Fig. 4). The observation relates to tlie time occu- 

 pied by the needle in returning to its position of equilibrium in the 

 meridian, after having been deflected into the east and west positions, 

 and Plateau found that in the case of water more time was required 

 when the needle was just afloat than when it was wholly immersed, 

 whereas in the case of alcohol the 

 time was greater in the interior. 

 The longer time occuj)ied when 

 the needle is upon the surface of 

 water is attributed by Plateau to 

 an excessive superficial viscosity of 

 that body. 



Instead of a needle, I have 

 here a ring of brass wire (Fig. 6), 

 floating on the surface of the water. 

 You see upon the screen the image 

 of the ring, as well as the surface 

 of the water, which has been made 

 visible by sulphur. The ring is so 

 hung from a silk fibre that it can 

 turn upon itself, remaining all 

 the while upon the surface of the 



water. Attached to it is a magnetic needle, for the purpose of giving 

 it a definite set, and of rotating it as required by an external magnet. 

 On this water, which is tolerably clean, w^hen the ring is made to 



Fig, 5. 



Fig. 6. 



turn, it leaves the dust in the interior entirely behind. That shows 

 that the water inside the ring offers no resistance to the shearing 



