VISCOSITY OF WATER BY THE EFFLUX METHOD. 119 
the uncertainty in the value of FR, so that the radius is # (1 + &), 
we have 
fR(1+€é)}* = Bt(1+4é+...) 
The other elements in a measurement are involved only to the 
first power, and therefore their determinations need be only to 
one fourth the precision of that required in the case of the radius, 
perhaps the most difficult of them all to accurately measure. 
III.—DeErERMINATION OF THE VISCOSITY. 
17.—General—Units.—C.G.8. units have been used thronghout, 
and the viscosity is expressed in absolute measure, .e. it is related 
to pressures expressed in dynes per square centimetre. 
Density of Water—The relative densities of water are derived 
from Mendelejeff’s table based on his formula! 
rie ATS) 
(94:1 +7) (703-51 —7)1°9 
Pr denoting the relative density at the temperature 7 centigrade. 
The absolute density at 4° C. has been taken as 1:000013 
grammes per cubic centimetre. 
pr=l 
Density of Mercury’—The density has been computed on the 
assumption that its value at 0° C. relative to water at 4°C. is 
135956, and its absolute density at 0°—grammes per cubic centi- 
metre—therefore 13-5958. The density at other temperatures has 
derived through Broch’s value for the coéfficient x of total 
cubic dilatation, viz. 
_X= (181808 +0-175 + +.0-035125 72) 10... eeeeeeeee (31) 
* being the temperature in Regnault degrees? z.e. the 1/100 part 
of the expansion 0° to 100° C. ‘The volume for the temperature 
; —— Russian Physical and Chemical Society, No. 5, 1891. 
Prati ‘i - ormation concerning the density of mercury at 0° C. see Traité 
37 Pa tau, thermométrie de précision: par Ch. Ed. Guillaume, p. 36, 
‘s aris 1889, 
“Sar veer et Mémoires du Bureau International des Poids et 
following ; wie - The expansions 0° to 100° C. are according to the 
tia... authorities Bosscha -018241, Wiillner ‘018253, Levy ‘018207 + 
Ve coéfficients give -018217. 
