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MESSES. FLEEMING JENKIN AND J. A. EWING ON FEICTION 
One part of Rennie’s work is of special interest on account of the light which has 
been thrown upon it by the comparatively recent progress of scientific theory. He 
observed that between ice and ice and between ice and steel (the temperature of the 
atmosphere being about 28° Fahr.) the coefficient of friction diminished largely as the 
pressure increased. The friction was far from regular ; but the diminution of the coeffi- 
cient under increased pressure was very marked, and amounted in one case to a change 
from 0T25 to 0-018. This result, which seems to be in complete accordance with 
J. Thomson’s discovery of the effect of pressure in lowering the melting-point of ice 
(Trans. R. S. E. 1849), does not appear ever to have had the attention directed to it 
that it deserves. 
Our most definite knowledge of the phenomena of friction is due to Mobin, who 
executed from 1830 to 1834 an elaborate series of experiments, the results of which were 
communicated to the Paris Academy of Sciences, and published in four memoirs. Mobin 
introduced the system of automatically registering the motion of a body along a hori- 
zontal plane surface under the action of certain forces, one of which was the friction to 
be measured, and he applied this method with great care to the determination of the 
friction between a large number of different substances. Fie not only confirmed the 
laws enunciated by Coulomb, but showed that all the numerous exceptions which 
Coulomb had mentioned conformed to the general laws when tested by the new and more 
accurate methods. The results of Mobin have been accepted as conclusive, and the 
work of subsequent experimenters has been practical rather than directly scientific in 
its object. 
Mobin agreed with Coulomb in distinguishing between static and kinetic friction, and 
although he did not observe that the time of rest affected the result to nearly so great 
an extent as Coulomb had affirmed, he found that the static value was usually greater 
than, but sometimes sensibly equal to, the kinetic. He also noticed that in many cases 
a slight shock was enough to destroy the distinction between the two. 
It occurred to us that instead of there being an abrupt change from the static to the 
kinetic value of friction at the instant in which motion begins, there might possibly be 
continuity between the two kinds, and hence that in those cases in which the static 
coefficient considerably exceeded the kinetic, the latter would be affected by changes of 
the velocity when the velocity was very small, in such a way as to increase as the velocity 
diminished. The experiments of Mobin showed that this change in the kinetic value, 
if it took place at all, must have been confined to very low velocities, so low that his 
method of observation did not enable him to detect it. The question of whether the 
friction is affected by changes of velocity under a velocity of, say, 0-01 foot per second, 
is left by the researches of Mobin an entirely open one ; for the length of time which 
elapsed in his experiments between the instant at which motion began or ended and 
that at which the velocity was only 0-01 foot per second must have been far too short 
to allow any definite measurement to be made during it of the rate of acceleration of the 
moving body. 
