780 
MR. H. TOM LINS OX ON THE COEFFICIENT OF VISCOSITY OF AIR. 
The spheres were attached to the suspenders S, K, in the same manner as the 
cylinders, but the disks were now dispensed with. The moment of inertia of the 
whole vibrator when the spheres were on was 30,927 in centimetre-gramme units, the 
vibration-period was 2'8791 seconds, and the temperature of the air and the baro¬ 
metric pressure were 9°’97 C. and 29 ’607 inches respectively. The uncorrected 
logarithmic decrement due to the friction of the air on the spheres was ’0003462, and 
the corrected logarithmic decrement was ’0003483. 
In deducing the value of p from the above data by the aid of equations (4), (o), 
and (6), I assumed, in finding 2/,’M / , a value for p equal to the mean of that got from 
the other experiments ; this step is admissible, because 2&M' is very small compared 
with I. # Having determined the value of k' by means of equation (4), I substituted 
it in equation (6), and thus obtained a quadratic equation for finding p. The 
quadratic may, however, be converted into a simple equation by making use of the 
same value of p as above in calculating the term - \/ACL which was thus found to 
n ° cc V 77 p 
be O’l 6085. The last number is not small compared with unity, and, had the final result 
proved to be as much as 10 per cent, greater or less than the mean of those got from 
the other experiments, the above conversion of the quadratic into the simple equation 
would not have been admissible. It will be seen eventually, however, that the 
conversion is legitimate, and the value of p at a temperature of 9°’97 C. as deter¬ 
mined from the simple equation is 
•00019334. 
Mathematical Formulce required for the Effect of the Rotation of the Spheres or 
Cylinders about their own AxesA 
Professor G. G. Stokes has been good enough to furnish me with the following 
formulae for the corrections not yet made for the effect of the rotation of the spheres 
or cylinders about their own axes :— 
Let be the logarithmic decrement due to the rotation, then for the spheres 
o ° ° 
o i\ r' vet A-6 -—To 
2pM t va 2(var 
l P 
l+~ + 
1 
logio e, 
va 2 (va)~ 
(?) 
where I is the moment of inertia of the whole system, r is the time of a vibration 
* In fact, is quite neg’lectable in the case before us. 
T What follows was added Sept. 16, 1886. 
