148 
MR. RENNIE’S EXPERIMENTS ON THE FRICTION AND 
periments 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 machinery 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 fths 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 required to over 
come the friction ought to be equal to produce that effect, whatever be the 
weight of the stone. 
3rd. That this force ought rather 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. 
5th. 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 1c Pont dc Nemours. 
•}■ L’Art de Batir. Tome iii. 1808. 
