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°-{~. 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 xx th 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 iii. 
-(- Memoire sur le Cintrement et Decintrement des Fonts. 
J Morisot, Tome iv. 
§ Recueil d’Experiences et d’Observations &c. 
