AND ITS APPLICATION TO MR, B. TOWER’S EXPERIMENTS. 
161 
brass the mean thickness of the him of oil would be sensibly constant, and hence, if 
the viscosity was constant, the resistance would increase directly as the speed. As 
this was not in accordance with Mr. Tower’s experiments, in which the resistance 
increased at a much slower rate, it appeared that either the boundary actions became 
sensible or that there must have been a rise in the temperature of the oil which had 
escaped the thermometers used to measure the temperature of the journal. 
That there would be some excess of temperature in the oil film on which all the 
work of overcoming the friction is spent is certain; and after carefully considering 
the means of escape of this heat, it seems probable that there would be a difference of 
several degrees between the oil bath and the film of oil. 
This increase of temperature would be attended by a diminution of viscosity, so 
that as the resistance and temperature increased with the velocity the viscosity 
would diminish and cause a departure from the simple ratio 
6 . In order to obtain a quantitative estimate of these secondary effects, it was 
necessary to know exactly the relation between the viscosity and temperature of the 
lubricant used. For this purpose an experimental determination was made of the 
viscosity of olive oil at different temperatures as compared with the known viscosity 
of water. From the results of these experiments an empirical formula has been 
deduced, by means of which definite expressions have been obtained for the approxi¬ 
mate variation of the viscosity with the speed and load. Taking these variations of 
viscosity into account, the results obtained from the hydrodynamical theory are 
brought into complete accordance with these experiments of Mr. Tower. Thus we 
have not only an explanation of the very novel phenomena brought to light by these 
experiments, and what appears to be an important verification of the assumptions on 
which the theory of hydrodynamics is founded, but we also find, what is not shown in 
the experiments, how the various circumstances under which the experiments have 
been made affect the results. 
7. Two circumstances particularly are brought out in the theory as principal 
circumstances which seem to have hitherto entirely escaped notice, even that of 
Mr. Tower. 
One of these is the difference in the radii of the journal and of the brass or bearing. 
It is well known that the fitting between the journal and its bearing produces a 
great effect on the carrying power of the journal, but this fitting is rather supposed to 
be a matter of smoothness of surface than a degree of correspondence in radii. The 
radius of the bearing must always be as much larger than that of the journal as is 
necessary to secure an easy fit; but more than this, I think, has never been suggested. 
Now it appears from the theory that if viscosity were constant the friction would 
be inversely proportional to the difference in radii of the journal and the bearing, and 
this although the arc of contact is less than the semicircumference. Taking the 
temperature into account, it appears from the comparison of the theoretical results 
with the experimental that at a temperature of 70 - 5° Fahr. the radius of one of the 
MDCCCLXXXVI. Y 
