BETWEEN SURFACES MOVING AT LOW SPEEDS. 
surfaces are moving at a moderate speed), the latter gradually increases when the 
velocity becomes extremely small, so as to pass without discontinuity into the former. 
The experiments of Coulomb show that the friction between surfaces at rest is itself not 
constant, but increases as the time of rest is prolonged. It seems doubtful whether 
this result is due to a real change in the static coefficient or not ; but if it is we may 
suppose that not only does the coefficient of friction increase continuously as the state 
of the surfaces changes from motion to rest, but continues for a time to increase after 
the latter state has been reached. To prove this with certainty is probably impossible, 
both because of the difficulty of observing the rate of retardation of moving surfaces 
down to the point at which motion wholly ceases, and because of the impossibility of 
measuring the static coefficient between surfaces which have not been in contact for a 
finite length of time ; but the results of the experiments which have been stated above 
seem to give a strong colour of probability to the hypothesis. 
The friction of steel on greenheart wetted with water (Case XII.) has special interest. 
Mokin has endeavoured to explain the excess of the friction of rest over that of motion, 
by supposing that the unguent present is more or less expelled when the surfaces are at 
rest. This view may perhaps in some cases be correct, but it is certainly not always 
tenable. With wetted steel and greenheart surfaces the static coefficient is very much 
greater than the kinetic, although the presence of water has an effect the reverse of 
that of an unguent (compare X. & XII.). It seems impossible that the expulsion of 
what has the effect of increasing the friction should give rise to a further increase. 
We found that our apparatus was not suited for determining the value of the friction 
between surfaces at rest. Attempts were made to use it for this purpose, but the results 
were not satisfactory. The static coefficients showed a fair general agreement with 
those given above ; the most marked divergences appeared in the cases where the kinetic 
values increased at low speeds. In these the static values of the coefficients were consi- 
derably greater than even the greatest kinetic values. 
In making practical deductions from experiments on friction, it is to be observed that 
the phenomena are so dependent on apparently insignificant variations of the conditions 
of the observation that it is hazardous to state, for the guidance of the engineer, results 
which are obtained under conditions greatly different from those met with in practice. 
We therefore only draw attention to two points, both of which seem to be novel : — 
(1) The excellence of greenheart, whether dry or oiled, as a material for bearings ; and 
(2) the very small friction between steel and brass when the surfaces are wetted with 
water. 
We regret that the laboriousness of the measurements and calculations which have 
had to be made in connexion with each observation has prevented the number of cases 
examined from being more numerous than they are. For the same reason, the experi- 
ments, of which the above is an account, although for the most part made in the summer 
of 1876, have only now been prepared for publication. 
Table II. gives a synopsis of the results. In Column IV. the value of p is given, 
