On Friction. 295 



€qual to the pressure of the weight W, the resulting equations 

 sTiow more clearly the circumstances which have place durnig the 

 descent, and also point out another correction. For the depth 

 of descent, x, does not increase directly as the force, and inverse- 

 ly as area and the modulus of elasticity. Let a=the area, and 

 w= the weight of the modulus of elasticity ; then d : — . Now 

 when this value of d is substituted in the preceding equations, it 

 is clear that the friction is not as the pressure, nor mversely as 

 the area, except just when the body is first moved from rest. 

 Consequently, these principles again lead to results which Pro- 

 fessor Vince had ascertained by experiments * ; viz. that the fric- 

 tion of a body in motion increases with the area of the body, and 

 in a less ratio than the pressure. ht d • » 



Mr. Meiklenext alludes to some experiments of Mr. Kennie s, 

 but I have not seen any of his experiments that have the least 

 connexion with the subject. 



When Mr. M. recollects that the time is a given quantity, or, 

 in other words, that the quantity of friction produced in a given 

 time is the thing to be estimated, his confused notion of the past 

 and present part of a given time will perhaps be removed. 



And lastly, I think it will appear very clear to your correspon- 

 dent, that I have not committed an error in estimating the fric- 

 tion at the axis of a wheel, when he considers that the space 

 moved through increases with the radius of the axle as well as 

 the velocity of the moving parts ; therefore, enlarging or dimi- 

 nishing the' axis does not aifect the resistance of t'le rubbing sur- 

 faces, and in this respect my conclusion remains correct. 



It may be necessary to remark, that the results that I have ob- 

 tained are not contradictory ; they are deduced from the known 

 laws of the cohesive force and extensibility of bodies, and the more 

 minutely they are investigated the more correctly they agree with 

 experiments. It will also be recollected, that it was my object 

 to obtain simple practical rules ; and that I did not profess to 

 give an accurate analysis of the laws of Friction. The method 1 

 have adopted is simple, and may be easily followed by those who 

 are acquainted with the mechanical properties of matter and 

 their laws of action ; but it is useless for any one to offer random 

 objections to any thing they do not understand. 



I am. Sir, yours, &c. 



Thomas Tredgold. 



• Philosophical Magazine, vol. xvii. p. 47- 



T 4 XLIX. Some 



