VISCOSITY OF WATER BY THE EFFLUX METHOD. 87 
a circle of radius R, and the pressure in a length L of the tube. 
fall from p, to p,, the volume q flowing in a unit of time across 
any right section of the tube is expressed by the equation :— 
T(Po—- Ps) 4 : 3 
G2 aa i: (R* +46 R*) 
in which &? is a constant determined by the internal resistance 
of the fluid, b one depending on the friction at its boundary, and 
x is 314159 etc. In recent treatises on hydrodynamics’ the 
equation is usually written 
q = “RP (Rea 4t eee (1). 
y, or Helmholtz’s k?, being the coéfficient of viscosity, and B that 
of sliding friction. This latter form is the earlier and the better, 
and is essentially that deduced by Neumann some time prior to 
1860. The reciprocal of the viscosity, f = 1/7, has been called 
the coéfficient of fluidity and 1/8 that of slipping. 
Limitation of the Equation of fluz.—The movements of 
liquids may be ciassed under two régimes, a fact clearly recognized 
by Darcy? in 1857, and since illustrated in a striking manner by 
the experiments of Reynolds? in 1883, and confirmed by those of 
Dometic! in 1888. In the first régime no eddies are formed, and 
the liquid, if limpid, has what has not inaptly been called a 
“plate-glass” appearance. In the second, eddies or vortices are 
lanes and the « sheet-glass” appearance may be noted, the 
‘Tefraction of the liquid being varied by the eddies. It is to flow 
‘under the first régime, that is to flow without vortices, that the 
1Ki 
Bante oo iiber mathematische Physik (1883 edit. dit.) p. 373. 
reatise on Hydrodynamics, 1888 Edit. Vol. 11.,p. 305. Lamb— 
tp his pf or ree ike of fluids, 1879 Edit. p. 224. Lamb uses p/p for 
- Matical cous, ett’sy, being what has been called by Maxwell the kine- 
cient of viscosity, i.e. 1 /p, p denoting the density of the fluid. 
twa relatives au mouvement de l’eau dans les 
857. See also Mém. des Savants étrangers t. 15, p. 215 
go 
emacs Vol. 174, p. 935 - 982. 
de Chimie, 6 Sér. t. 21, p. 464. 
