160 



MR. RENNIE'S EXPERIMENTS ON THE FRICTION AND 



Appendix to Tables VII. and VIII. 



Table showing the coiriparative amount of Friction of different Metals under 

 an average pressure of from 54.25lbs. to 69.5olbs. as calculated from the 

 foregoing experiments. 



Description of Metals. 



Average Weight. 



Proportion. 



Weight per Square 

 Inch Area. 



Brass on wrought iron , 



Steel upon steel 



Brass upon cast iron , 



Brass upon steel , 



Hard brass upon cast iron .... 

 Wrought iron on wrought iron , 



Cast iron upon cast iron 



Cast iron upon steel 



Cast iron upon wrought iron. . , 



Tin upon wrought iron 



Brass upon brass , 



Tin upon cast iron . . . .- 



Steel upon wrought iron 



Tin upon tin , 



lbs. 

 69.55 

 69.55 

 54.25 

 69.55 

 54.25 

 69.55 

 54.25 

 69.55 

 69.55 

 69.55 

 69.55 

 54.25 

 69.55 

 69-55 



7.312 

 6.860 

 6.745 

 6.592 

 6.581 

 6.561 

 6.475 

 6.393 

 6.023 

 5.846 

 5.764 

 5.671 

 5.198 

 3.305 



lbs. 

 11 

 11 



8 

 11 



6 

 11 



8 

 11 

 11 

 11 

 11 



8 

 11 

 11 



oz. 

 12.4 

 12.5 



0.5 

 12.5 

 15.9 

 12.5 



0.5 

 12.5 

 12.5 

 12.5 

 12.5 



0.5 

 12.4 

 12.5 



Remarks cm Tables VII. and VIII, 



1. From the preceding experiments it appears: — that the friction of metals varies with their 

 hardness. 



2. That the hard metals have less friction than the soft ones. 



3. That without unguents and within the limits of 32lbs fioz. per square inch, the friction of hard 

 metals against hard metals may very generally be estimated at about one-sixth of the pressure. 



4. That within the limits of their abrasion the friction of metals is nearly alike. 



5. That from 1.66cwt. per square inch to upwards of 6cwt. per square inch, the resistance increases 

 in a very considerable ratio, being the greatest with steel on cast iron, and the least with brass on 

 wrought iron, their limits being as 30, 36, 38, and 44cwt. An experiment was made with a weight 

 of 10 tons per inch on hardened steel, which abraded. 



The remarkable property of steel in hardening, and its power to resist abrasion, render it preferable 

 to every other substance yet discovered in reducing the friction of delicate instruments, as is exem- 

 plified in tlie different experiments on the pendulum, and the assay and other balances recently in- 

 troduced at His Majesty's Mint and the Bank of England. 



The experiments of Messrs. Cavendish and Hatchett in the years 1798 and 1801 at His Ma- 

 jesty's Mint on the alloys, specific gravity, and comparative wear of gold coin by friction, likewise 

 prove that friction and abrasion were less in the hard than sofl metals. Philosophical Transactions 

 for 1803, Part I. 



