168 MR. rennie's experiments on the friction and 



XIII. On the Friction of Stones. 



RoNDELET found that stones well dressed required angles of from 28° to 36° 

 before they commenced gliding*. Perronet observed them to vary from 39° 

 to 40° -f-. The granite voussoirs of the arches of the New London Bridge 

 having their beds well faced and dressed without mortar, generally commence 

 gliding at angles of from 33° to 34°. But with a bed of fresh and finely 

 ground mortar interposed, the pressure on the centring commences at angles 

 of from 25° to 26°. In other cases of arches where sandstones, such as Bramley 

 Fall and Whitby were employed, and their beds faced and dressed as usual, 

 the angle of gliding was found to vary from 35° to 36°. But with mortar 

 interposed the angle generally varied from 33° to 34°. 



It results from these and other experiments, that friction, by absorbing part 

 of the horizontal thrust, is a most powerful assistant in maintaining the equili- 

 brium of arches, and enables us to determine with something like precision 

 the allowances due to theory. 



In general, stones which have a fine grain and uniform texture, and are 

 sonorous and heavy, resist abrasion in proportion to their hardness; and in some 

 experiments of Morisot;}:, granite resists abrasion twelve times more than lias, 

 whilst the former only possesses a repulsive power three times greater than the 

 latter. 



The experiments of Boistard give 0.78 for the friction of hard calcareous 

 stones §. 



XIV. On the Friction of Machines. 



1. 21cwt. (suspended at each extremity of a chain passing over two cast 

 iron sheaves of 2 feet diameter with wrought iron axles, working in brass bear- 

 ings oiled, and 12 feet 10 inches apart) was disturbed by 3 cwt. or xVth of the 

 total weight. Another double purchased crane indicated ^th, 



2. A double purchased crane having a weight of 7057lbs. suspended to it 

 indicated 7-62 for the friction. Another double purchased crane indicated ^th. 



• L'Art de Batir. Tome hi. 



f Memoire sur le Cintrement et Decintrement des Fonts. 



% Morisot, Tome iv. 



§ Recueil d'Experiences et d'Observations &c. 



