308 Scientific Intelligence. 
the negative; this was developed and fixed by the usual pro- 
cesses. Even when an interval of a number of hours had elapsed 
after the exposure, the light from the calcium sulphide surface 
was still intense enough to produce the effects described. 
ll. The Coefficient of Viscosity of Air; by Hersperr Tommin- 
son.—In the previous experiments by the author on this subject 
the coefficient of viscosity of air was determined from observa- 
tions of the logarithmic decrement of amplitude of a torsionally 
vibrating wire, the lower extremity of which was soldered to the 
center of a horizontal bar. From the bar were suspended verti- 
cally and at equal distances from the wire a pair of cylinders, or 
a pair of spheres. The distances of the cylinders or spheres from 
the wire were such that the matn part of the loss of energy 
resulting from the friction of the air may be characterized as 
being due to the pushing of the air. A small part, however, of 
the whole logarithmic decrement was due to the rotation of the 
spheres or cylinders about their axes, and Professor Stokes has 
kindly added to the paper a note in which are deduced formule 
which serve to correct for this effect of rotation. 
Acting on a suggestion of Professor Stokes, the author pro- 
ceeded to determine the coefficient of viscosity of air by suspend- 
ing a hollow paper cylinder about 2 feet in length and 4 foot in 
diameter, so that its axis should coincide as to its direction with 
the axis of rotation. The cylinder was supported by a light hol- 
low horizontal bar, about 7 inches in length, to the center of 
which the vertically suspended wire was soldered. The wire was 
set in torsional vibration, and the logarithmic decrement deter- 
mined with the same precautions as before. 
The mode of eliminating the effect of the internal friction of 
the metal wire, and also the effect of the air on the ends of the 
cylinder, is fully described in the paper. 
The following were the results :— 
Vibration-period Coefficient of viscosity Temperature in degrees 
in seconds. of air, /. Centigrade. 
3°6038 0°00017708 12°225 
8°8656 0:00017783 13°075 
_ In these experiments the loss of energy arising from the friction 
of the air may be characterized as being due to the dragging of 
the air, and it is very remarkable that there should be such close 
agreement in the values of « as determined by this and the pre- 
vious methods. The mean value of the coefficient of viscosity of 
air obtained by this method is 0:00017746 at a temperature of 
12°650° C., and the mean value deduced from the previous ex- 
periments when proper correction has been made for the rotation 
of the spheres and cylinders about their axes is 0°00017711 at a 
temperature of 11°79° C. The above values of are given in 
C.G.S. units.— Proc. Roy. Soc., No. 248. 
12. Note on Barus and Strouhal’s paper on the Viscosity of 
Steel; by W. F. Barrert, Professor of Physics, Royal College 
