An important part of the investigation was to ascertain whether the friction is increased by the velocity of the rubbing bodies. With respect to the bodies- of the same kind descending on inclined planes, Coulomb found that the time re quired for passing over the first half was a little more than double that required for passing over the second. But a body put in motion by a constant accelerating force employs for passing over one space, and over two equal consecutive spaces, times that are to each other in the ratio of .11 : .12=100 : 142 ; that is to say, if 100 units of time are consumed in pass ing over the first space, 142 will be con sumed in passing over the first and se cond together, and consequently 42 in passing over the second. Now this agrees as nearly as possible with the re sult of the experiments ; consequently we infer that a load drawn along a smooth plane by a constant accelerating force (that of a descending weight for exam ple) is uniformly accelerated. But this requires that the friction, at every in stant, destroys only a proportional quan tity of the force added by the constant action of gravity. The conclusion there fore is, that for moderate velocities at least, the resistance due to friction is a constant quantity, and very nearly the same for every degree of velocity.
Another point of great importance was to ascertain the relation the friction bears to the for example, in what ra tio the friction has increased by doubling or trebling the load. Coulomb found that when wood has been allowed to rest on wood for some time, without the in tervention of any unguent, the resistance occasioned by the friction is proportional to the pressure. The resistance for a short time increases rapidly by the con tact, but attains its maximum in a few minutes. The friction of wood sliding on wood with any velocity is still propor tional to the pressure ; but the resistance is much less in amount than that which is required to detach the surfaces after some minutes of contact. In the else of oak, for instance, the force required to detach the surfaces after being sonic mi nutes in repose is to that which is neces sary to overcome friction alone after mo tion has commenced in the ratio of 100 to 23. The friction of metals on metals is also proportional to the pressure ; but the intensity is the same, whether the surfaces have been any length of time in contact, at rest, or are gliding along with a uniform velocity.
The friction of heterogeneous sub stances, as woods and the metals, is en tirely different from the above. In the case of wood against wood dry, or of me tal against metal, the friction of the rub bing bodies is very little influenced by the velocity ; but in the present case the friction increases very sensibly with an augmented velocity. Coulomb in
ferred that the friction increased as the natural numbers, when the velocities are increased as the squares of those num bers. In all cases of a hard body rub bing against a very soft substance, the friction increases remarkably with the velocity.
Since the friction is in general propor tional to the pressure, it follows that it will not be altered by increasing or di minishing the extent of the rubbing sur faces. ]Nevertheless, this consequence fails in the extreme cases. The friction is sensibly diminished when the surfaces in action are reduced to the smallest di mensions. Thus, while the friction of a ruler of brass against a similar one of iron is expressed by 2G, it was found to be only 17 after the sledge had been mounted on 4 round-headed brass nails. Other causes of friction are the rough nesses, spiculve, and angles of surfaces removeable by filling them with oil-tal low, Ste., and by diminishing the extent of surface in contact. Olive-oil reduces the friction of woods one-half.
In all cases, the rubbing of large Bur-. faces against each other should be avoid ed; and hence the use of litta wheels to turn with the axis of shafts called friction wheels, by which the contact and rub bing of large breadths is avoided. Differ ent substances, too, should work against one another, the ultimate atoms of the bodies tending to combine by their simi larity of forms.
In the screw and the wedge, the fric tion is equal to the power. The sheaves of pullies should not press against the blocks.
It has been carefully determined at Baltimore, that one quart of oil is suffi cient for 2000 miles run of a steam-car riage weighing 3 tons. In the Winan's waggon, the friction-wheels dip into the oil ; but being in a cast-iron case, none is lost, while the renewal of oil is better than the same in long work. Purified ve getable oils answer best.
Black-lead is found to destroy friction with the best effect.
Ferguson found that the quantity of friction was always proportional to the weight of the rubbing body, and not to the quantity of surface ; and that it in creased with an increase of velocity, but was notproportional to the augmenta tion of celerity. lie found also, that the friction of smooth soft wood, moving up on smooth soft wood, was equal to one third of the weight ; of rough wood upon rough wood, one half of the weight ; of soft wood upon bard, or hard upon soft, one-fifth of the weight ; of polished steel upon polished steel or pewter, one quar ter of the weight ; of polished steel upon copper, one-fifth ; and ofpolished steel upon brass, one sixth of the weight. Coulomb brought to light many newand strikfng phenomena, and confirmed others, which were previously hut par tially established.