PUBLIC BUILDINGS. 



361 



on by the carbonic acid contained in the atmo- 

 sphere ; if argillaceous, as in many kinds of gray- 

 wacke, it will also be readily decomposed, and the 

 stone become a mass of sand and clay. 



The Pyramids of Egypt are built of granite, 

 though some travellers say of a grayish calcareous 

 stone containing shells. The Parthenon and other 

 public buildings of Athens, which have withstood 

 the action of the elements during the lapse of more 

 than 20 centuries, are of Pentelic marble. The 

 mosques of Constantinople are of a fine-grained 

 limestone, similar to that which is now used in li- 

 thography. The Coliseum, and St. Peter's Church 

 at Rome, are built of travertino, a calcareous stone 

 deposited from water. The ruins of Paestum are 

 built of the same. The building called the tomb 

 of Theodric, at Ravenna, has a dome of limestone, 

 consisting of a single stone 34 feet in diameter, 

 and weighing more than two million pounds. The 

 houses in Paris are chiefly built of a calcareous 

 stone. St Paul's, and many of the public buildings 

 of London, are constructed of oolite, called Port- 

 land stone, a calcareous rock which we have al- 

 ready described. Most of the ancient statuary is 

 formed of Parian marble. The pedestal of the 

 statue of Peter the Great, at St. Petersburgh, 

 weighing three million pounds, is of granite. Six- 

 ty granite columns of St. Petersburgh consist each 

 of a single stone 20 feet high. Pompey's Pillar, in 

 Egypt, is 63 feet in height, composed of a single 

 piece of granite. 



ENGINEERING. 



An accurate knowledge of the physical and chem- 

 ical properties of materials is no less important 

 to the engineer than to the architect. Indeed, as 

 he is employed in the construction of public works, 

 such as railroads, canals, aqueducts, roads, bridges, 

 embankments, quays, dikes, sea-walls, &c, whose 

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