Hennessy — On the MoJecular Influcucc of Fhiids. 55 



YI.— On Expeei:\iei7ts to Determine the Intltjekce op the Molectjlae 

 Condition of Flitids on thelr Motion when in Rotation, and in 

 Contact with Solids. Eeport, Part I. By Henry Hennessy, 

 P. R. S., M. R. I. A., Professor of Applied Mathematics in the 

 Royal College of Science for Ireland. 



[Eead, February 12, 1877.] 



Observation and experiment have long since completely established 

 that no fluid can be considered as strictly exhibiting the physical pro- 

 perties of the ideal substance commonly defined in almost all treatises 

 and class-books on hydro-mechanics as a perfect fluid. 



In a perfect fluid, the particles are supposed to be free to move 

 among each other by the smallest application of force — in other 

 words, they are supposed to be absolutely smooth, and totally des- 

 titute of cohesion. AVith such qualities, their motions would be 

 free from friction amongst themselves, and almost free from friction 

 when in contact with many solid substances. The practical experience 

 of hydraulic engineers has long since clearly shown, that the flow of 

 liquids is accompanied with very notable resistances, which observers 

 attributed to the friction of the liquids against the surfaces of the 

 vessels and pipes in which they were moving. More recently, mathe- 

 maticians and physical inquirers have been led to recognise a kind of 

 resistance to the motion of fluids depending entirely on the molecular 

 properties of the fluids themselves, and to this kind of resistance the 

 name internal friction has been generally applied. If this term is to 

 continue in use, it appears to me that care should be taken to clearly 

 discriminate between this kind of resistance to motion and that of 

 friction, as it is commonly understood between solid bodies. If two 

 perfectly smooth surfaces of solids of the same fonn and molecular char- 

 acter are in close contact^ as, for instance, two sheets of glass, such 

 great resistance to the motion of one upon the other exists, that 

 portions of them are liable to shear away. The resistance to motion 

 in this case is almost entirely due to cohesion. If two dissimilarly 

 constituted substances be moved in coiitact, experience shows that 

 when they are very smooth the motion meets with little resistance, but 

 when they are rough, the resistance may be very considerable. In 

 the latter case work must be expended in causing the asperities of one 

 surface to surmount or nib away the asperities of the other, and 

 hence, in some languages, friction is spoken of as synonymous with 

 attrition. We possess no evidence for assuming that such conditions 

 exist between the particles of a fluid as to justify us in classifying the 

 internal resistance to its motion as identical with this kind of force. 

 If we reflect on the phenomena of the molecular statics of liquids pre- 

 sented in the well-known phenomena of capillary attraction, and in 

 the admirable researches of Plateau on masses of liquid free from the 



