MR. H. TOMLINSON ON THE COEFFICIENT OF VISCOSITY OF AIR. 
779 
Experiment IV. 
Acting on the advice of Professor Stokes, I modified Experiment III. as follows :— 
The logarithmic decrement was determined with the paper cylindeis already used, and 
also with another pair of the same diameter, and made in the same manner, but having 
a length of 7‘700 centims., the vibration-period being by the usual device maintained 
very nearly the same in both cases. The difference between the two logarithmic 
decrements, '0024564 and '0009933, will therefore equal the logarithmic decrement 
due to the resistance of the air on cylinders having each a length of (60'875—7'700) 
centims., i.e., 53'175 centims. When the longer paper cylinders were on the bar the 
vibration-period was 2'9994 seconds. The temperature of the air and the barometric 
height were 10 c '64 C. and 30'057 inches respectively. The uncorrected logarithmic 
decrement was '0014631, and the corrected logarithmic decrement '0014638. The 
value of p, at the temperature of 10°'64 0., deduced from the above data, wms 
•00017955. 
Experiment V. 
The previous experiments had given such closely according values of /x that, though 
my investigations on the internal friction of metals only required that the formuke 
for cylinders should give consistent results, I felt that it would be of interest to 
ascertain whether the use of spheres would be attended with the same satisfactory 
agreement. The main difficulty to be encountered with spheres is that the mass of a 
properly constructed spherical shell makes it rather unsuitable for experiments on the 
viscosity of gases. After thinking over various plans of obtaining hollow spherical 
shells of sufficiently accurate make, and not feeling satisfied that I should be able to 
get, without much difficulty, what I wanted, I decided on using solid spheres made of 
fairly light wood. These spheres were specially turned for me, with instructions to 
make each as exactly as possible 2p inches in diameter. The turner executed his 
commission very fairly, for, on gauging each sphere at ten different places with calipers 
reading to xcfooth of an inch, I found that none of the readings differed from the 
mean by so much as '3 per cent., and that the mean diameters of the two spheres 
were 2'5103 inches and 2'5007 inches respectively. In the calculations each sphere 
was reckoned as having a diameter of 2'5055 inches or 6'364 centims. The masses of 
the two sjffieres were not quite so equal as I could have wished, the apparent mass of 
one in air being 64'823 grammes, and of the other 63'761 grammes. No appreciable 
error will, however, be introduced by considering the apparent mass of each in air to 
be 64’292 grammes. The correction for the mass of air displaced by each sphere 
amounted to 0*168 gramme, so that in the calculations the mass of each sphere was 
taken as 6 4 '4 60 grammes. 
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