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XXIII. On the Friction and Resistance of Fluids. By George Rennie, Esq. 
V.P.R.S. 
Read June 16, 1831. 
When on a former occasion I communicated the results of a series of 
experiments on the Friction and Resistance of the Surfaces of Solids (Philo- 
sophical Transactions for 1828), I stated that they formed part only of a series 
of experiments on the nature of friction generally. My object at first was to 
trace the relation subsisting between the retardation produced by the surfaces 
of solids in motion when in contact with each other and with fluids ; but 
finding that the subject connected with either of these branches was sufficiently 
extensive, I deemed it necessary to postpone the second part of the inquiry to 
a future occasion. Those experiments, however, established some important 
facts. They showed that (within the limits of abrasion) friction was the same 
for all solids, and that it was neither affected by surface nor velocity. Subse- 
quent experiments upon rolling bodies of great weight and magnitude, when 
the resistance was reduced ToVoth part of the mass, and the surfaces in the 
ratio of 13 to 1, have corroborated the affinity of resistance between rolling 
and sliding bodies. Thus in connecting and continuing the isolated experi- 
ments of Coulomb and Vince, and assigning values to the abrasive resistances 
of most of the most useful solids, a considerable advance has been made in the 
science. 
The subject of the present paper, however, involves difficulties of a more 
complicated kind. The theory of solids as deduced from the laws of mechanics, 
and independent of experiment, may be applied to any system of bodies ; but 
the theory of fluids, in which the form and the disposition of the particles, or 
the laws of their action, are unknown, must necessarily be founded on experi- 
ment ; and even with this aid, which can only be obtained through the inter- 
vention of a solid, our knowledge of the true properties of fluids must be vague 
3 i 
MDCCCXXXI. 
