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 miUion 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. Petersburghy 
weighing three miUion 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 smgle 
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-w^alls, &c., whose 
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