574 
MR. W. C. D. WHETHAM ON THE ALLEGED SLIPPING 
Tuhe No. 5 was then cleaned with acid and amalgamated by leaving it for some 
time filled with mercury, and running a stream of mercury through several times. 
The radius was re-determined, r — -0833 cm. 
Average pressure. 
Temperature. 
Time 
of flow. 
Value of /t. i 
cnis. 
/ 
// 
3'36 
15-4 
10 
56 
•01324 
3-.35 
1.5-5 
10 
54-5 
•0I3I7 
4-94 
15-7 
7 
12 
•01282 
In the experiments with the same tube described above, when the surface was 
copper, the following results were obtained at similar pressures : — 
Average; pressure. 
Temperature. 
Time of flow. 
Value of /«. 
cms. 
O 
3-U7 
16-6 
11 
33 
•01293 
3 93 
16-6 
8 
59 
•01288 
5-00 
16-6 
6 
54 
•01258 
Thus in none of these experiments does the value of p, differ much from that given 
by PoiSEUiLLE for glass tubes, but, like his, agrees with the formula deduced from 
the supposition that no slip occurs. In all cases it is slightly greater, which is readily 
explained by irregularities iu the tubes, owing to the difficulty of drawing them. 
According to Girard’s results, the value of p should have about a quarter of the value 
given by Poiseuille, but in none of the exjieriments described in this paper did it 
fall below Poiseuille’s value, and more decisive still, no change in the nature of the 
surface changed the rate of flow ; this is purely a comparative metliod, and seems 
much more reliable than the absolute method of Girard, which depends on accurate 
measurements of the radii of the tubes, differences in pressure, &c. Girard only used 
tubes of two sizes, and gives no account of the means he employed to estimate their 
radii. At the same time it should be noticed that his values for the t\\o sizes agree 
fairly between themselves with the supposition that a slipping coefficient exists, 
whose value is about 0'4 mm. Any constant error in the estimation of the radius, 
would however be naturally of greater importance in the smaller tube, and may ha\’e 
led to the apparent agi'eement with the results of an effect, inversely proportional to 
the radius, and due to the existence of a finite slipping coefficient. 
We must now return to the consideration of the experiments of Helmholtz and 
PlOTROWSKI. 
The discussion of the body of their paper I must leave to those with the requisite 
mafhematical knowledge, merely observing in passing that it is remarkable that the 
value they deduce for the coefficient of viscosity of the liquid itself is considerably 
