274 TIMEHRI. 
by smaller or succour drains; 10 or 12 feet is sufficient 
for most main drains, but if the soil is low, or loose, a 
greater width will be required to make the exterior dam 
of sufficient strength. The best form of these dams is 
that of a military field work, but with the ditch inside 
instead of outside. No dam or trench should be without 
its parapet, which in a dam answers to the parapet of a 
field work, and in a trench to the covered way. The 
parapet of a dam is a support to the foundation. That 
of a trench, by taking off for the breadth of 2 to 4 feet 
from the outside of the trench, the earth from 1 to 2 feet 
deep, for the whole length, preserves the trench from 
being filled up by the falling in of the sides. One is 
astonished to see the immense length of drainage re- 
quired, but the ease with which a trench is opened in 
a fat moist clay, without a single impediment of rock or 
stone, renders it a labour of great facility. The most 
faulty part of the drainage consists in the improper con- 
struction of the sea dams in which no attention is paid to 
those principles which in Europe are commonly praétised 
with the greatest success. Almost the whole length of 
coast is at different periods liable to an accumulation of 
surf, that destroys the dams, and in many instances 
washes away great traéts of land from the front of 
estates. A dam is generally repaired by driving piles in 
rows along the beach, facing the sea, and carrying earth 
to fill up the breach. It is well-known to Engineers in 
Europe, that in many instances even a strong wall of 
brick or stone is insufficient to stand the swell of the sea 
when opposed to it without any intermediate gradual 
check. The breakwater at Plymouth, having a base of 3 
times its height and 6 times its breadth at top, would be 
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