294 THE FINE ARTS. 



Durability of Granite. 



Some granites resist for ages the destroying efforts 

 of the weather, while others are resolved into sand 

 and clay in a comparatively short period. The 

 obelisk at the Place de St. Jean de Lateran, at Rome, 

 was quarried at Syene, under the reign of Zetus, 

 king of Thebes, 1300 years before the Christian era, 

 and that in the Place de St. Pierre, also from Egypt, 

 has resisted the elements for 3000 years. But be- 

 tween the two extremes of great durability and rapid 

 decay, there are numerous intermediate degrees ; for 

 in the same mountain, and even in the same hillock, 

 granites of very different qualities are met with. 



Strength of Masonry. 



Brickwork, composed with Parker's, Lord Mul- 

 grave's, or Pozzolana cement, will bear 23 tons 

 weight on each superficial foot before the bricks will 

 crack, and 50 tons before it will be totally crushed. 

 Portland stone of the best quality will not split with 

 less than 173J tons; and a bedding or joint of 

 Pozzolana mortar is not destructible with that 

 weight. Aberdeen granite resists the greatest 

 weight that can be put upon it, whereas all other 

 granites are crushed by their own superincumbent 

 weight when they exceed 200 feet in altitude. 



Waterloo Bridge. 



Waterloo bridge is built of Cornish granite, ex- 

 cept the balustrades, which are of granite from Aber- 

 deen. The three old bridges are built of Portland 

 stone, and are rapidly decaying. Alternate freezing 

 and thawing greatly injure stone exposed to the 

 river. The price of a cubic foot of stone is doubled 



