56 Proceedings of the Royal Irish Academy. 



action of gravity, we must conclude that, although the intermobility of 

 the molecules is the fundamental property of all liquids, this property 

 is accompanied by a considerable amount of cohesion. "When liquids 

 possess cohesion in a high degree, they are said to be viscid. As far 

 as my observations have gone, I have been led to conclude that the 

 resistance to liquid motion, designated as internal friction, arises 

 almost entirely from cohesion of their particles, and that it might be 

 more simply and more correctly called viscidity} 



The paramount influence exercised by the motions of fluids in 

 many of the most remarkable phenomena of nature has long since in- 

 duced observers to undertake experimental researches with reference 

 to the resistances of fluids. Coulomb" was the first who made syste- 

 matic experiments with a view to elucidate the influence of cohesion 

 on the resistance of fluids. His researches appear to me to have been 

 chiefly directed to the examination of the resistances experienced 

 between the surface of a moving solid and a fluid in contact. Similar 

 experiments have been more recently performed by Meyer.^ Helm- 

 holtz* and Pietrowzki have carried on experiments with a notably 

 modified method. 



In the two first of these inquiries the experiments were conducted 

 by causing a solid horizontal circular disk immersed in the fluid to 

 undergo periodical oscillations around a vertical axis. In the third, the 

 fluid was included in a solid globe, which was subjected to periodic 

 oscillations. In order to deduce from the facts of observation in these 

 cases any results as to the external or internal resistance of the fluids, 

 elaborate mathematical theories are indispensable. In the latter case 

 especially, the oscillations of the hollow globe produce waves whose 

 motions complicate the phenomena so as to require complex formulae 

 for their expression. 



Among the phenomena in which the internal resistance of fluids 

 may be important, those where liquids are in rotation about an axis 

 seem to be more simple than where they are undergoing large oscilla- 

 tions, and it thus occurred to me that an experimental study of such 

 a motion of fluids might prove fruitful in results. From the nature of 

 fluids it seems likely, whatever precautions we may take, that even 

 the most simple kinds of motion of their particles cannot take place 

 with great rapidity without tlie production for a time of some kind of 

 oscillation. But the results will probably be less complicated than 



^ The floatation of small solids of greater density than the liquids on the fluid 

 surface shows that the cohesion of the particles of liquid among themselves may be 

 sometimes accompanied by repulsion for solids. I have already brought imder the 

 notice of the Academy some remarkable phenomena of this kind, in which the co- 

 hesion of the liquid particles for each other was clearly illustrated, as well as their 

 repulsion for the small solid bodies. — " Proceedings of the Royal Irish Academy," 

 vol. i., series ii., p. 153. 



- " Memoires de I'lnstitut National," torn. iii. 



3 Poggendorfs " Annalen," vol. cxiii., p. 55. 



* " iSitzungsberichte der kais. Academic zu Wicn," vol. xl. 



