Wwe have from (6) 
VISCOSITY OF WATER BY THE EFFLUX METHOD. 99 
corrective term whose factor is 1/7, the other part being also a 
constant. Hence (6) may be written 
Coro (6a). 
y a 
To determine C, the number must be found towards which PZ’ 
approaches when 7’ is increased without limit, and therefore P 
diminished without limit. The equation 
PT =C+—7 (6b). 
is that of a straight line, such that if the several values of 1/7’ be 
regarded as abscisse and the corresponding values of PT as 
ordinates, the line passing through the points so determined will 
intersect the axis of ordinates at the distance C from the origin, 
and make with the axis of abscisse an angle whose tangent is ¢. 
This value C corresponds io the value 1/7’=9, or T=, the 
corrective term therefore vanishing.| Fig. 2 shews the plot for 
Poiseuille’s tubes A, A, B, and C;. Should the products PT shew 
no progressive increase, as is sometimes the case,” no value can be 
assigned to c, nor consequently to m, for, when c is found, m is 
determined from (6) and (6a) by the equation 
m= 6 ae ia ree (7) 
for absolute units. If the pressures be ees by the lengths in 
centimetres of the columns of liquid in the manometers, h, it will 
be generally convenient to form the products h 7’ instead of PT. 
Let » denote the density in grammes per cubic centimetre of the 
liquid in the manometers, and g the acceleration of gravity in 
centimetres per second, then from the relation 
P=pg = phg 
ae fe ig! Ae ig ae -as-(8) 
: 4 no + 
and writing 3 idee gh Me, 
ae 
1 
Ao a zero when 1/T = 0, or T= %. 
£0 great es of the members of the series may be, and often are, 
ee obliterate, as it were, all evidence of the progression. — 
