4 On the Nature and Laws of Friction. 
of machinery, a certain portion of the moving force is lost in 
friction ; in some cases nearly the whole, and in all it is consi- 
derable. The importance therefore of a correct mode of calcu- 
lating the effect of friction is perhaps one of the greatest desi- 
derata in mecnanical science. 
The greater part, if not the whole, of our knowledge respect- 
ing friction, is the result of experimental inquiry; and the few 
investigations that have been attempted have been conducted as 
if it depended wholly on the internal structure of bodies: at least 
such is the principle adopted by Parent and Belidor*, who con- 
sidered bodies to be composed of spherical particles, and that 
friction consisted in the force lost in raising these spherical pro- 
tuberances over one another, But were that the case, there could 
not be a difference in the friction of different bodies. ‘The most 
ingenious and elegant illustration of the nature of friction is that 
given by Professor Leslie (in his Inquiry into the Nature of Heat, 
&c.) The Professor very justly observes that “its existence de- 
monstrates an unceasing mutual change of figure, the opposite 
planes during the passage continually seeking to accommodate 
themselves to all the minute and accidental varieties of contact. 
The one surface being pressed against the other becomes, as it 
were, compactly indented by protruding some points and re- 
tracting others. This adaptation is not accomplished instanta- 
neously, but requires very different periods to ‘attain its maszi- 
mum, according to the nature and relation of the substances 
concerned” 
I will now endeavour to exhibit, in its most simple form, the 
relation between the friction and those properties of bodies by 
which it is affected, in hopes that it may tend to elucidate the 
subject; and also furnish some valuable hints respecting the na- 
ture of the bodies that are best adapted to move on one another 
with the least friction. 
The form of the surfaces of bodies is determined by their in- 
ternal structure ; and whatever degree of smoothness may be 
given by art, it is impossible to render those surfaces perfect 
planes ; and when two bodies are pressed together, the surfaces 
in contact will, in proportion to the force, become indented into 
one another. Whatever may be the depth of indentation, when 
the body is caused to slide, a part equal to this depth is supposed 
to be abraded or torn away. ‘That such an abrasion does take 
place, is apparent from common observation, as all bodies wear 
when exposed to friction. There are, however, some very flexible 
bodies that by yielding escape being abraded, such as caoutchouc, 
&c. which appear to require an extension of these principles to 
include them, 
* Architecture Hydraulique. 
Assuming 
