158 GEOLOGY OF THE COMSTOCK LODE. 



as compared with the sheet adjoining it on the west. The consideration of 

 a sheet or plate of rock under the influence of friction of a relatively 

 opposite character on its two faces, therefore, forms the natural starting 

 point for an examination of the observed conditions. 



Friction a force. — What is callcd frictlon^ is a complex phenomenon which 

 has never been satisfactorily reduced to a mathematical expression, and is 

 perhaps incapable of such a reduction. It is usually regarded as a mere 

 resistance, a force to which the negative sign is indissolubly attached. Pro- 

 fessor Reuleaux- has insisted upon the incorrectness of this view and has 



' It is generally considered that the sensible movements, say of a rough block of stone dragged 

 over a pavement, are of the same character as those involved in the friction of smoother surfaces. On 

 the larger scale it is plain that projections of the moving body will meet those ot the underlying sur- 

 face, and exert a pressure upon them precisely as iu the case of the teeth of gearing. When the 

 draught has reached a certain intensity, and when the points of contact are small surfaces, approxi- 

 mately normal to the direction of translation, the projections on one or the other surface will give way, 

 and heat will result. If the areas of actu.al contact are small surfaces, inclined at a considerable angle, 

 the moving body will rise to surmount them. In falling again a portion of the energy of position will 

 be converted into heat by the impact, but as all bodies arc to some extent elastic, the energy of position 

 will not all be dissipated. 



If a block of griinite is at rest upon a pavement, it assumes the lowest possible position, the max- 

 imum number of i>oints of contact are established and the projections on the two surfaces overlap to 

 the greatest possible extent. Wheu the same block is set in motion, the energy imparted to it prevents 

 its settling into maximum contact. 



It is plain that the resistance of the block will be greatest at the moment when motion begins, 

 or th.at the so-called friction of rest is somewhat in excess of the friction of motion. It would .also 

 seem that the friction of rest is merely the maximum value of the friction of motion, and such is the 

 result of the recent investigations of Messrs. Jenkin & Ewing. The greater the velocity of the moving 

 body the less thoroughly will the projections ot the two surfaces interlock; on the other hand, 

 points which at a low velocity would meet one another nearly in vertical lines, will at high velocities 

 meet on a line considerably inclined, and the horizont.al component of the elastic force developed by 

 impact will act as a resistance. Morin took the elasticity of carriage springs into consideration iu deter- 

 mining the resistance of a pavement to the passage of vehicles. It appears to me that it must also 

 enter into the true expression for the coiifficient of friction. The excess of the friction of rest over that 

 of motion is evidently due iu part to the fact that when at rest the energy of position which must be 

 overcome is at a maximum, while after motion has set in a portion of this energy is elastically returned 

 to the moving body. Besides those elements of friction which have been mentioned, adhesion also 

 undoubtedly plays a part, at least in the case of very smooth surfaces. 



The following deductions from the experiments of Coulomb and Morin are approximations only : 



(1.) Friction is proportional to the pressure normal to the contact of the rubbing surfaces. 



(2.) It is independent of their extent. 



(3.) It is independent of their velocity. 



According to Rankin the excess "of friction of rest over the friction of motion is instantly de- 

 stroyed by a slight vibration." A vibration of course develops the elastic force. 



The friction of lubricated surfaces appears to me wholly different from that of dry ones. A shaft 

 should not come in direct contact with its bearing, and the work done would seem to consist iu a very 

 active stirring of a thin layer of oil. The amount of this work will be dependent on the adhesion of 

 the lubricator to shaft and bearing as well as upon the geometrical character of the solid surfaces. 



^The Pneumatics of Machinery, by F. Reuleaux, translated by A. B. W. Kennedy, p. 595. The 

 translator states that similar views are mniiitaimd in Bell's Experimental Mechanics, a work I have not 

 met with. 



