844 HistOTy of Mechanical Inventions and 



Granite on granite, - - - 3.30 



Yellow deal on yellow deal, - - 2.88 



Sand-stone on sand-stone, - - 2.75 



Woollen cloth on woollen cloth, - - 2.30 



These results are collected from the different Tables, but the 



comparison may be made by selecting other values within the 



limits of abrasion for a minimum. 



General Conclusions. 



From what has been stated hitherto it is obvious, — 



1st, That the laws which govern the retardation of bodies 

 gliding over each other are as the nature of those bodies. 



2c/, That with fibrous substances, such as cloth, &c. fric- 

 tion is increased by surface and time, and diminished by pres- 

 sure and velocity. 



2d, That with harder substances, such as woods, metals, 

 and stones, and within the limits of abrasion, the amount of 

 friction is as the pressure directly, without regard to surface, 

 time, or velocity. 



Aith, That with dissimilar substances gliding against each 

 otlier, the measure of friction will be determined by the limit 

 of abrasion of the softer substance. 



5t/i, That friction is greatest with soft, and least with hard 

 substances. 



6th, That the diminution of friction by unguents is as the 

 nature of the unguents, without reference to the substances 

 moving over them. 



The very soft woods, stones, and metals, approximate to 

 the laws which govern the fibrous substances. 



In comparing the present experiments with those of Cou- 

 lomb, the discordances found to exist relate principally to time. 

 The limited pressures (varying from 1 to 45lbs. per square 

 inch) under which his experiments were made, account in some 

 degree for the anomaly. But in many of the minor, and in 

 the general results, they will be found to coincide. — PhiL 

 Trans. 1829, p. 169- 



3. On an Indelible Ink. By M. Henui Braconnot. 



To 20 grammes of Dantzic potash dissolved in boiling water, 



