FRICTION. 
of the surface to which they are to be 
adapted. But when other circumstances 
remain the same, the difficulty of bending 
a rope increases with the square of its dia- 
meter, as also with its tension ; and it de- 
creases according as the radius of the cur- 
vature of the body to which it is adapted 
increases. Of the simple mechanical pow- 
ers, the lever is the least subject to friction. 
In a wheel, the friction upon the axis is, 
as the weight that lies upon it, as the dia- 
meter of the axis, and as the velocity of the 
motion. But upon the whole, this sort of 
tfiction is not very great, provided the ma- 
chine is well executed. In common pul- 
leys, especially those of a small size, the 
friction is very great. It increases in pro- 
portion as the diameter of the axis in- 
creases, as the velocity increases, and as 
the diameter of the pully decreases. With 
a moveable tackle, or block of five pulleys, 
a power of 150 pounds wall barely be able 
to draw up a weight of 500 pounds. The 
screw is subject to a great deal of friction ; 
so much so, that the power which must be 
applied to it, in order to produce a given 
effect, is at least double that which is given 
by the calculation, independent of friction. 
But the degree of friction in the screw, is 
influenced considerably by the nature of 
the construction, for much of it is owing to 
the tightness of the screw, to the distance 
between its threads, and to the shape of 
the threads ; the square threads producing, 
upon the. whole, less friction than those 
which are sharp. The friction which at- 
tends the use of the wedge, exceeds, in ge- 
neral, that of any other simple mechanical 
power. Its quantity depends so much upon 
the nature of the body upon which the 
wedge acts, besides other circumstances, 
that it is impossible to give even an ap- 
proximate estimate of it. The friction of 
mechanical engines not only diminishes the 
effect, or which is the same thing, occasions 
a loss of power; but is attended with the 
corrosion and wear of the principal parts 
of the machine, besides producing a consi- 
derable degree of heat, and even actual 
fire ; it is, therefore, of great importance, 
in mechanics, to contrive means capable of 
diminishing, if not of quite removing, the 
effects of friction. 
The methods of obtaining the important 
object of diminishing the friction, are of 
two sorts, viz. either by the interposition 
of particular unctuous, or oily substances 
between the contiguous moving parts, or by- 
particular mechanical contrivances. Olive- 
nil is th« best, and perhaps the only sub- 
stance that can be used in small works, as 
in watches and clocks, when metal works 
against metal. But in large works the oil 
is liable to drain off, unless some method 
is adopted to confine it. Therefore for 
large works tallow is mostly used, or grease 
of any sort, which is useful for metal, as 
well as for wood. In the last case tar is 
also frequently used. The mechanical con- 
trivances which have been made, and are 
in use, for the purpose of diminishing the 
effects of friction, consist either in avoid- 
ing the contact of such bodies as produce 
much friction, pr in the interposition of rol- 
lers, viz. cylindrical bodies, between the 
moving parts of machines, or between mov- 
ing bodies in general. Such cylinders de- 
rive, from their various size arid application, 
the different names of rollers, friction 
wheels, and friction rollers. Thus in mill- 
work, and other large machines, the wooden 
axis of large wheels terminate in iron gud- 
geons, which turn in wood, or more fre- 
quently in iron or brass, which construction 
produces less friction than the turning of 
wood in wood. In the finest sort of watch- 
work the holes are jewelled, viz. many of 
the pivots of the wheels, &c. move in holes 
made in rubies, or topazes, or other hard 
stone, which, when well finished, are not 
liable to wear, nor do they require much 
oil. In order to understand the nature of 
rollers, and the advantage with which their 
use is attended, it must be considered, that 
when a body is dragged over the surface 
of another body, the inequalities of the sur- 
faces of both bodies meet and oppose each 
other, which is the principal cause of the 
friction or obstruction ; but when one body, 
such as a cask, a cylinder, or a ball, is 
rolled upon another body, the surface of 
the roller is not-robbed against the other 
body, but is only successively applied to, or 
laid, on the other, and is then successively 
lifted up from it. Therefore, in rolling, 
the principal cause of friction is avoided, 
besides other advantages : hence a body 
may be rolled upon another body, when the 
shape admits of it, with incomparably less 
exertion than that which is required to drag 
it over the surface of that other body. In 
feet, we commonly see large pieces of tim- 
ber, and enormous blocks of stone, moved 
upon rollers that are laid between them 
and the ground, with ease and safety, when 
it would be almost impossible to move them 
otherwise. 
Friction, is a term made use of in 
medicine, and implies the act of rubbing a 
diseased part with oils, or other substances. 
