EMBANKMENTS. 
365 
ing his estimates of the expense. In such cases, 
it is evident that a knowledge of rocks on the sur- 
face is not sufficient ; he must know what rocks he 
at the different depths to which the canal must be 
cut, and to this end he must be acquainted with the 
order of succession of the different strata. 
Embankments, — In the construction of roads and 
canals, walls and embankments often have to be 
erected ; and from the frequent repairs needed on 
our canals from the shding of embankments, we ap- 
prehend they are often constructed without regard 
to those rules and principles, on the observance of 
which durabihty alone depends. Sir Henry Par- 
nell remarks, that " great care is necessary in ma- 
king high embankments ; no person should be in- 
trusted with these works w^ho has not had consid- 
jerable experience as a canal or road maker ; for if 
the base of an embankment is not formed at first to 
its full breadth, and if the earth is not laid on in 
regular layers or courses not exceeding four feet 
in thickness, it is almost certain to shp. 
'* There have been but few attempts to make em- 
bankments by turnpike trustees that do not afford 
illustrations of this defect, and of a want of knowl- 
edge of the proper rules by which these works 
should be managed. No doubt a chief reason for 
making cuttings and embankments, as is frequently 
the case, with slopes of one to one, has been to 
save expense in the purchase of land and moving 
earth ; but the consequence of making such slopes 
is, that the earth is constantly slipping, so that, in 
the end, the expense is always greater in correct- 
ing the original error than it would have been had 
proper slopes been made in the first instance. The 
slopes at which cuttings and embankments can be 
safely made, depend entirely upon the nature of the 
soil. In the London and plastic clay formations, it 
will not be safe to make the slopes of embankments 
or cuttings that exceed four feet in height^ with a 
Hh 2 
