90 G. H. KNIBBS. 
slipping does occur, so that Whetham’s conclusion must ie 
restricted. That it occurs in cases of flow in glass tubes hag 
certainly not been shown, but in regard to copper tubes the inter- — 
pretation of Girard’s experiments seemed to require that slipping 
be postulated,” and it is somewhat singular that a recent deter — 
mination by Brodmann of the value of 7/8 for water by the 
sphere and cylinder methods gives a result?—7/ = 0- 0409-—very 
nearly identical with Helmholtz’s value‘ = 0:03984—deduced from : 
the efflux observations of Girard. On the other hand it has Be ; 
pointed out by Jacobson that Helmholtz in criticising Girard’s 
experiments failed to apply any correction to the pressure, and : 
that his own—Jacobson’s—experiments with copper tubes lend 1 
no support to the opinion that slipping occurs.6 The general 
theory of flow in tubes being however beyond the scope of this 4 
discussion, it will suffice to show that Helmholtz’s view is unsup | 
ported by experimental results in cases of steady rectilinear * 
in glass tubes, such as are used in observations of efflux having 
for their object the determination of the viscosity. 
If we put 
. _ TP - pps 
1) 2 
anda = 46 = te 
then we shall have from Be by transposition, 
7 = m (1+ =) 
the term a/R depending on the existence and influence of sliding | 
friction, so that, if the viscosity be computed without reference? 
thereto, we shall obtain the results 7» instead of », and these will: 4 
show a variation with widely different values of /, since ais 
constant. 
From the results hereinafter quoted, obtained from Poiseuilles 
experiments on the assumption that slipping does not occur WO 
1 Wied. Annal. Bd. 45, p. 184. oie | 
2 As against this view see Couette’s memoir Annal. de Chimie, 6 9% : 
t. 21, 1890, p. 492 - 494, : 
' § Wied. Annal. Bd. 45, p. 179. 
* Sitzber. Wien. Akad. Bd. 40 40, p. 656. 
5 Achiv. fiir Anat. u. Phys., 1861, p. 306. 
