It should likewise be observed that, in order to
secure the boyaux from being enfiladed, the an-
gles which they form are to be less open than in
the attack of polygons, and therefore, a greater
number of boyaux must be constructed ; which
of course augments the labour of the besieger.
134 ON THE ATTACK AND

and consequently retards his progress. It is cal-
culated that a fortification of the nature of that
above described, is capable of holding out '50
days at least, from the opening of the trenches ;
and that the garrison may defend it 60 days, by
making a proper use of covered batteries.
133. — When several fronts are constructed
upon a concave curve, they are considered as be-
ing actually unattackable ; for, if the besieger
should direct his attacks against one of the fronts
at the extremities of the curve, the fronts, at the.
