AND ITS APPLICATION TO MR. B. TOWER’S EXPERIMENTS. 
1G9 
at temperatures from 95° to 200° Fahr. ; in the second set, from 61° to 120° Fahr. 
In so far as the temperatures overlapped, the viscosities for the two oils agreed to 
within 4 per cent., but as the law of variation of the viscosity seemed to change 
rapidly at about 140° Fahr., only the second set have been recorded. These are shown 
in Table I. 
From Poiseuille’s experiments it is found that, measuring viscosity in pounds on 
the square inch, for water at a temperature of 61° Fahr., 
ft=10" 7 X P61. 
Adopting this value of /x for the experiments on water at 61° Fahr., the other 
experimental values of /x for water at different temperatures, obtained as being in the 
ratios of the times, were found to be in very close agreement with those calculated 
from Poiseuille’s law for the respective temperatures. This tested the efficiency 
of the apparatus. It has not been thought necessary to record any experiment on 
water except at the temperature of 61° Fahr. 
The experimental value of /x for oil are in the ratios of the times multiplied by 
'915, the specific gravity of oil ; these are given as the experimental values of /x in 
the table. Another column contains the values of /x for oil, calculated from an 
empirical formula fitted to the experimental values. 
This formula was found by comparing the logarithms of the experimental values 
of /x. It appeared that the differences in these logarithms were nearly proportional 
to the differences in the corresponding temperatures, or that T being temperature in 
degrees Fahr., 
lo g>i— lo g /r 3 =-0096(T 3 —TJ 
in degrees Centigrade 
log/Xi— log/x 3 ='00535(T 3 —T x ) 
whence since 
*0096=-0221 log 10 e 
•00535 = -0123 log 10 e 
for degrees Fahrenheit 
for degrees Centigrade 
-g—0221(Ti-T s ) 
ffi-g—0123(T,-T 2 ) 
/^2 
( 2 ) 
This ratio holds well within the experimental accuracy from temperatures ranging 
from 61° to 120° Fahr. This is shown in the table, and again in fig. 3, in which 
the ordinates are proportional to log /x, the abscissse being proportional to the 
corresponding temperatures. 
MDCCCLXXXVI. Z 
