MR, H. TOMLINSON ON THE COEFFICIENT OF VISCOSITY OF AIR. 
783 
Table II. — Cylinders. 
Number of 
experiment. 
Length in 
centime. 
Diameter in 
eentims. 
Distance 
between the 
centres in 
eentims. 
Vibration-period 
in seconds. 
Temperature in 
degrees 
Centigrade. 
Coefficient of 
viscosity of air 
in C.G.S units. 
I. 
60-875 
2-5636 
20-80 
6-8373 
12-02 
•00017900 
II. 
60-885 
0-9636 
20-80 
7-0590 
14-63 
•00017680 
III. 
60-875 
2-5636 
20-80 
3-0198 
11-69 
•00017767 
IV. 
53-175 
2-5636 
20-80 
2-9994 
10-64 
•00017581 
Spheres. 
V. 
6 - 364 
20-78 
2-8811 
9-97 
•00017626 
Taking the means of the numbers in the sixth and seventh columns, we find that 
the value of /u, at a temperature of 11°* * * § 79 C. is 
•00017711. 
The Effect of the Presence of Aqueous Vapour on the Viscosity of Air. 
The above experiments extended over a period of some months, during which the 
air was in various conditions with respect to being saturated with aqueous vapour, so 
that for a rough approximation we may assume that the mean value for \x just given 
will apply to air half saturated with vapour at a temperature of 12° C., and it would 
appear that the presence of the small quantity of aqueous vapour which this implies 
would not affect the value of y to an extent equal to that of the probable error in 
experimenting. From the careful investigations of Mr. Crookes* we learn that at a 
temperature of 15° C., and under pressures of from 760 to 350 millims., the presence 
of aqueous vapour has little or no influence on the logarithmic decrement. By 
the aid of Professor Stokes’s note,+ I have estimated that at 15° C., and under a 
pressure of 760 millims., the air when saturated with aqueous vapour would be more 
viscous than perfectly dry air| to the extent of only “2 per cent. It is not until the 
air is under a less pressure than 350 millims. that the aqueous vapour begins to show 
appreciable effect, but when the rarefaction is great the moist air becomes considerably 
less viscous than dry air. 
According to Maxwell § damp air over water at a temperature of 21°T1 C., and 
under a pressure of 101 millims., is less viscous than dry air by about -g^th part. 
* ‘ Phil. Trans.,’ Part II., 1881, p. 427. 
t See p. 440 of tire above paper. 
I Mr. Ckookes adopted great precautions to render the air dry. 
§ ‘ Phil. Trans.,’ vol. 156, 1866. 
