358 
ARCHITECTURE. 
Without multiplying instances, we may refer to the 
aqueduct at Rochester, which is already in a state 
of dilapidation from exposure to the elements ; and 
we could name a splendid public building, erected 
from the crumbling marble of Singsing, which 
promises an equally short-lived existence. It is a 
dictate of common sense, that a knowledge of the 
general structure of rocks, and the situations whence 
the best materials may be obtained, is essential to 
the builder and such as are charged with the erec- 
tion of public works. A few hints on this subject 
are all we have space to offer. 
A stone which is sufficiently durable underwater 
may not be so when kept alternately wet and dry by 
the rise and fall of water in a river, or canal, or on 
a tidal coast, or even when wholly exposed to the 
atmosphere. If it be a porous sandstone, it will 
probably last well if constantly under water ; but, 
if exposed to the air, the same rock will be certain 
to crumble in climates subject to frost. Owing to 
inattention to this simple fact, many of the finest 
national structures of England, where there abounds 
an almost inexhaustible variety of rocks of the most 
imperishable quality, have gone to premature decay. 
It is painful," says Philips, " to mark the injuries 
effected by a few centuries on the richly sculptured 
arches of the Normans, the graceful mouldings of 
the early English architects, and the rich fohage of 
the decorated and later Gothic styles. The chan- 
ging temperature and moisture of the air, communi- 
cated to the slowly conducting stone, especially on 
the western and southern fronts of buildings, bursts 
the parts near the surface into powder, or, by intro- 
ducing a new arrangement of the particles, separ- 
ates the external from the internal parts, and causes 
the exfoliation or desquamation of whole sheets of 
stone parallel to the ornamental work of the mason 
From these attacks no shelter can wholly protect 
the parts of a building which are below a ledge of 
