Mr. Vince adds some remarks on the na- 
ture of the experiments which have been 
made by others. These, he observes, the au- 
thors “ have instituted to find what moving 
force would just put a body at rest in mo- 
tion and they concluded thence that the ac- 
celerative force was then equal to the fric- 
tion ; but it is manifest, that any force which 
will put a body in motion must be greater 
than the force which opposes its motion, 
otherwise it could not overcome it: and 
hence, if there were no other objections than 
this, it is evident that the friction could not 
be very accurately obtained; but there is an- 
other objection, which totally destroys the 
experiment, so far as it tends to shew the 
quantity of friction, which is the strong cohe- 
sion of the body to the plane when it lies 
at rest.” This he confirms by several expe- 
riments, and then adds: “From tliese ex- 
periments, therefore, it appears how very 
considerable the cohesion was in proportion 
to the friction when the body was in motion; 
. it being in one case almost y, and in another 
it was found to be very nearly equal to the 
whole friction. All the conclusions, there- 
fore, deduced from the experiments which 
have been instituted to determine the fric- 
tion, from the force necessary to put a body 
in motion, (and I have never seen any de- 
scribed but upon such a principle) have ma- 
nifestly been totally false; as. such experi- 
ments' only shew the resistance which arises 
from the cohesion and friction conjointly.” 
Philos. Trans, vol. 75, pa. lt>5. 
If a body is laid upon another body, and 
soon after is moved along the surface of it, 
a less force will be found sufficient for the 
purpose, than if the body be left some time 
at rest before it be moved. This arises prin- 
cipally from an actual change of figure, which 
is produced in a longer or shorter time, ac- 
cording to. the nature of the bodies. Thus 
the maximum of adhesion between wood and 
wood takes place in a few minutes’ time ; be- 
tween metal and metal it takes place almost 
immediately. A hard and heavy body laid 
upon a softer one will sometimes continue to 
increase its adhesion for days and weeks. 
When a cubic foot of soft wood of eight 
pounds weight is to be moved upon a smooth 
horizontal plane of soft wood, at the rate of 
three feet per second, the power which is ne- 
cessary to move it, and which is equivalent to 
the friction, amounts to between l-4th and 
l-3rd of the weight of the cube. When the 
wood is hard, the friction amounts to between 
J -7th and i-8th of the weight of the cube. 
In general the softer or the rougher the 
bodies are, the greater is their friction. Yet 
when two pieces of metal, extremely well 
polished, are laid one upon the other with an 
ample surface of contact, thev adhere to each 
other much more forcibly than when they 
are not so w ell polished. 
Iron or steel moves easiest in brass. Other 
metals acting against each other produce 
more friction. 
The friction, caeteris paribus, increases 
with the weight of the superincumbent body, 
and almost in the same proportion. 
The friction or obstruction which arises 
from the bending of ropes about machines, is 
influenced by a variety of circumstances, such 
as their peculiar quality, the temperature ot 
the atmosphere, and the diameter or curva- 
FRICTION. 
ture of the surface to which they are to be 
adapted. But when other circumstances re- 
main the same, the difficulty of bending a 
rope increases w ith the square of its diameter, 
as also with its tension, and it decreases ac- 
cording as the radius of the curvature of the 
body to w hich it is adapted, increases. 
Of the simple mechanical powers 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 diameter 
ot the axis, and as the velocity of the motion. 
But upon the whole, this sort of friction is 
not very great, provided the machine is 
well executed. In common pulleys, espe- 
cially those of a small size, the friction is very 
great. It increases in proportion as the dia- 
meter ot the axis increases, as the velocity- 
increases, and as the diameter of the pulley 
decreases. With a moveable tackle, or block 
of live pulleys, a power of 150 pounds will 
barely be able to draw up a weight of 500 
pounds. 
The screw is subject to a great deal of fric 
tion; so much so that the power which must be 
applied to it, in orderto 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 construc- 
tion ; 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 attends the use of the 
wedge, exceeds in general that of any other 
simple mechanical power. Its quantity de- 
pends, so much upon the nature of the body 
upon which the wedge acts, besides other 
circumstances, that it is impossible to give 
even an approximate 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 pro- 
ducing a considerable degree of heat, and 
even actual fire; it is therefore of great im- 
portance in mechanics to contrive means ca- 
pable of diminishing, if not of quite remov- 
ing, the effects of friction. 
In compound engines, the obstruction 
which arises from friction can be ascertained 
only by means of actual experiments. An 
allowance, indeed, may be made for each 
simple component mechanical power ; but 
the error in estimating the friction of any 
one single power is multiplied and increased 
so fast by the other parts, that the estimate 
generally turns out very erroneous. Besides, 
much depends on the execution of the work ; 
the quality of which cannot be learned but 
by experience. Novices are generally apt 
to expect too much or too little from any 
mechanism. In general it can only be said. 
775 
that in compound engines, at least one-third 
of the power is lost on account of the fric- 
tion. 
The methods of obtaining the important 
object of diminishing the friction, are of two 
sorts, viz. either by the interposition of par- 
ticular unctuous or oily substances between 
the contiguous moving parts, or by particular 
mechanical contrivances. 
Olive-oil is the best, and perhaps the only- 
substance that can be used in small works. 
as in w atches and clocks, when metal works 
igainst metal. But in large works the oil is 
liable to drain off, unless some method is 
adopted to coniine it. Therefore for large 
works t allow r 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. 
In delicate works of wood, viz. when a 
piece of wood is to slide into or over wood, 
and when a wooden axis is to turn into wood, 
the fine powder of what is commonly called 
black-lead, when interposed between the 
parts, eases the motion considerably, and is at 
the same time a clean and durable sub- 
stance. 
Though olive-oil is the best and the only 
substance that is used for delicate mecha- 
nisms; yet it is far from being free from ob- 
jections. Oil, when in contact with brass, is 
liable to grow rancid, in which state it slowly 
corrodes the brass. In different tempera- 
tures it becomes more or less fluid; but upon 
the whole it grows continually thicker, and 
of course less lit to ease the motion of the 
parts, &c. Trifling as those defects may at 
lirst sight appear, they are however of such 
moment in delicate works, that in the greatly 
improved state to which watchwork has been 
brought in this country, the changeable qua- 
lity of the oil seems at present to be the prin- 
cipal, if not the only impediment to the per- 
fection of chronometers. 
The mechanical contrivances which have 
been made, and are in use, for the purpose 
of diminishing the effects of friction, consist 
either in avoiding the contact of such bodies 
as produce much friction, or in the interpo- 
sition of rollers, viz. cylindrical bodies, be- 
tween the moving parts of machines, or be- 
tween moving bodies in general. Such cy- 
linders derive, from their various size and 
application, the different names of rollers, 
friction-wheels, and friction-rollers. 
Thus in mill-work and other large ma- 
chines, the wooden axes of large wheels ter- 
minate in iron gudgeons, which turn in wood, 
or more frequently 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 stoue, which, when well finished, are not 
liable to wear, nor do they require much oil. 
In order to understand the nature of rol-* 
lers, and the advantage with which their use 
is attended, it must be considered, that when 
a body is dragged over the surface of an- 
other body, the inequalities of the surfaces of 
hotly 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 bali, is rolled upon an- 
other body, the surface of the roller is not 
rubbed 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 incompa- 
rably less exertion than that which is required 
to drag it over the surface of that other body. 
In fact, we commonly see large pieces of 
t iimbeiyand enormous blocks of stone, moved 
