F R t 



opposition to friction, is as the square of the velocity communicated to 

 it. Thus a nail is driven by a blow of no great force, into a piece of 

 wood where the mere friction is sufficient to retain it against a great 

 force applied to draw it out. 



5. When motion begins, the intensity of friction diminishes j it does 

 not, however, change afterwards as the velocity changes, but continues, 

 as already said, to retard with a uniform force. Coulomb found the 

 friction of wood sliding on wood to become less when the body began to 

 move, than it had been the instant before in the ratio nearly of 2 to 9. 



6. Friction may be measured by placing the body on a plane of vari- 

 able inclination, and increasing that inclination till the body begin to 

 slide. If the weight of the body W, and the inclination of the plane 

 when the body begins to slide = 0, the friction W X tan. 0. 



7. Time is often required for friction to attain its maximum, and in 

 this respect different substances differ much from one another. 



8. Friction is diminished by unctuous substances ; those that are thin- 

 nest and least tenacious are the best ; plumbago reduced to powder, and 

 rubbed on the surface of wood, metal, stone, &c. serves greatly to di- 

 minish friction. 



9. The effect of friction may be diminished by drawing a body in a line 

 inclined at a certain angle to the plane on which it rests. Thus if the 

 weight of a body be to its friction on a horizontal plane as n to 1, it will 

 be drawn with the greatest ease in the direction which makes with that 



plane an angle, having for its tangent . 



10. The friction of cylinders rolling upon an horizontal plane is in a 

 direct ratio of their weights, and in the inverse ratio of their diameters, 



11. The momentum of friction is diminished by friction wheels in the 

 ratio of the radius of the axis of any one of the wheels (they are suppos- 

 ed equal) to the perpendicular height of the axis that rests upon them, 

 above the line joining their centres, 



12. In wheel carriages, the plane on which they move, and the line of 

 draught, being both horizontal, the advantage for surmounting an im- 

 movoable obstacle, of a given height, is as the square root of the radius 

 of the wheel. 



Let the whole weight to be moved be W, the radius of the wheel r, 

 f the force which drawing horizontally will raise the carriage over an 



immoveable obstacle of the height h s then/= W X V - . 

 127 H 



