558 TRANSACTIONS OF THE 



first complete set of experiments on this most important subject, 

 and they are of inestimable value to the practical engineer. 



Between similar substances, under like circumstances, friction 

 is a continuous retarding force. It is greatest between bodies the 

 surfaces of wliich are rough, and is much lessened by polishing 

 them. If the two surfaces that come together remain precisely 

 the same, the friction will, most certainly, increase directly, ac- 

 cording to the pressure ; and it will not be changed by diminish- 

 ing or increasing the surfaces that rub each other, except in ex- 

 tretue cases, when the surfaces are reduced to minute dimensions. 

 Culomb inferred that the frictioa increased as the natural num- 

 bers, when the velocities are increased as the squares of those 

 numbers. 



When metal brakes are used on railroad cars, the friction is but 

 slightly affected by motion; when wood is used to rub against the 

 wheel, it is necessary that the two bodies remain some time in 

 contact to enable the friction to acquire a maximum. If the 

 pressm-e is permitted to remain the same, the friction can have 

 no relation to the size of the sui'face. The friction of an iron 

 rail can brake on an iron wheel, is proportional to the pressure, 

 the intensity is the same, whether they have been long in contact, 

 or are moving with a uniform speed. It is different with two 

 wooden bodies, they must rest on each other for a long time, 

 without the use of grease, to occasion the resistance by friction, 

 to be proportional to the pressure. 



Much greater friction may be effected by using wooden bridles 

 for rail car brakes, than iron, as friction is greater with soft, than 

 hard substances. With metals, the friction is directly as the 

 pressure, without regard to surface or velocity. But with fibrous 

 matters, friction is much increased by time and surface, and de- 

 creased, by velocity and pressure. 



It is surprising to me that rail car, and locomotive wheels last 

 as long as they do, when I consider the continuous strain they 

 have to endure, a constant disturbance of the particles of iron 

 takes place, deteriorating and ultimately breaking them. 



Heating, from time to time, will consolidate these particles; but 

 the wheel never retm-ns to its original size; it constantly ac- 

 quires an increase of volume, as often as it is heated and cooled. 

 This maybe proved to you by examining the bars of our common 

 house grates, in which hot fires are made, they elongate for a 

 time, and then become curved. Four years since, I placed eight 

 iron bars in the furnace of my green house, in such a manner 



