170 
MR. RENNIE’S EXPERIMENTS ON FRICTION AND ABRASION. 
2nd. That with fibrous substances, such as cloth, &c. friction is increased 
by surface and time, and diminished by pressure and velocity. 
3rd. That with harder substances, such as woods, metals, and stones, and 
within the limits of abrasion, the amount of friction is as the pressure directly, 
without regard to surface, time, or velocity. 
4th. That with dissimilar substances gliding against each other, the measure 
of friction will be determined by the limit of abrasion of the softer substance. 
5th. That friction is greatest with soft, and least with hard substances. 
6th. That the diminution of friction by unguents is as the nature of the un- 
guents, without reference to the substances moving over them. 
V 
The very soft woods, stones, and metals, approximate to the laws which 
govern the fibrous substances. 
In comparing the present experiments with those of Coulomb, the discor- 
dances found to exist relate principally to time. The limited pressures (varying 
from 1 to 45lbs. per square inch) under which his experiments were made, 
account in some degree for the anomaly. But in many of the minor, and in 
the general results, they will be found to coincide. 
The subject might be illustrated still further by detailing the results of other 
experiments on the motions of machines, on the friction of solids revolving 
in fluids, and the descent of carriages down inclined planes. But as the pre- 
sent inquiry principally relates to the friction of attrition of solids, and as the 
experiments last mentioned have not been sufficiently matured to arrive at the 
necessary deductions, it only remains to conclude by expressing a hope, that 
the data now furnished will in some degree enlarge the bounds of our know- 
ledge on this subject, interesting as one of philosophical inquiry, and inti- 
mately connected with every branch of the mechanical arts. 
