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148 MR. RENNIE'S EXPERIMENTS ON THE FRICTION AND 



perimeiltk on the friction and resistance of heavy machinery. The results 

 varied under different circumstances ; but it appeared that an augmentation of 

 resistance took place in proportion to the quantity of machineiy put into action. 

 In one instance in the ratio of 1 to 5, when it absorbed from one-fifth to one- 

 tenth of the power expended. 



This anomaly, as compared with the ratio of surfaces in the present experi- 

 ments, can only be accounted for, from the irregularity of the movements and 

 the difficulty of producing simultaneous actions in complicated machinery ; the 

 more especially as the results were affected by contingencies which could not 

 be properly estimated ; some of the elements on which the deduction is founded 

 not being stated. The resistance was likewise increased by reversing the di- 

 rection of motion. The velocities being very moderate, and hardly exceeding 

 120 feet a minute, appeared to have had no influence: but the experiments 

 related principally to the resistances produced by different kinds of machinery. 

 The experiments of M. Boistard* on the gliding of stones with a view to de- 

 Velope the equilibrium of arches, led him to conclude that the relation of the 

 friction to the pressure was constant ; that asperity of surface did not alter 

 its value, which generally amounted to yths of the pressure. 



From similar experiments M. Rondelet concluded-}-, 



1st. That the rougher the surface of stones, the greater the power required 

 to move them. 



2nd. That the greater the insistent weight, the greater the resistance : but 

 as the inequalities are apt to be broken, the maximum force i*equired to over 

 come the friction ought to be equal to produce that effect, whatever be the 

 weight of the stone. " ' "^" '-■ ' i 



3rd. That this force totiptf' Itlther to be in the ratio of the hardness of the 

 stone than of its weight. 



'"'4th. The amount of friction varied from one-half to one-third of the in- 

 sistent weight. ' '^'J««« OJ f ' 

 >^-Vih. The angle of equilibrium of similar stones was about 30 degrees. And 



6th. Finally, extent of surface did not alter its value. 



The experiments of Morisot on the grinding and polishing of stones, and 

 of Maniel and Pasley on the pressure and equilibrium of earths, present 



. . • Recueil d'Experiences et d'Observations, &c. sur le Pont de Nemours. 

 ' '^'>£'^ 5"' .j- L'Art de Batir. Tome iii. 1808. 



