THE EOYAL ARTILLERY INSTITUTION. 
53 
of the top of the cover from the point of impact, and the less the 
u 
difference of height of these two points, the smaller will be -, and the 
flatter the requisite angle of descent. 
Now, in order to keep the distance a as large as possible, we must 
endeavour, in curved breaching and demolition fire, to take the scarps 
in lines down the ditches, not across them. We shall in this way 
obtain flat angles of descent when firing at the escarps, the flanks, 
the caponnieres opposite the centres of the sides of the polygon, those 
parts of the bastion faces that can be struck through the ditches of the 
ravelins, the redoubts in the covered way if they can be taken in the 
direction of its branches, &c., &c. In the case of the older fortresses, 
it will, however, often happen that the distance a is sufficiently great 
to give a flat angle of descent. 
The artifice of increasing the distance a by adopting an oblique line 
of fire, is, in the case of breaching and demolition fire against strong 
masonry, of little use; because what is gained, as regards striking 
force, by reducing the angle of descent (in • other words, by increasing 
the charge) is lost by the projectile striking obliquely. We therefore, 
in general, only make choice of an oblique position if other local cir¬ 
cumstances render it necessary. 
Finally, in order to reduce the value of the quotient -, it is important 
CL 
that the difference in height of the top of the cover and point of impact 
should be kept as small as possible. With given profiles this partly 
depends on how the point of impact is chosen. This will be fully dis¬ 
cussed further on; for the present we will simply allude to the means 
by which the profile conditions may be altered—viz., by knocking 
away the top of the covering mass, and by employing mines to par¬ 
tially remove it. 
It is only under exceptional circumstances that the height of the 
cover can be reduced more than 1 or 1*5 metres by knocking away its 
crest, even when 15 c.m. shells 2J calibres in length and heavy bursting 
charges are employed. It is, moreover, not always possible to attain a 
lower point of impact by knocking away the crest of the glacis, as may 
be seen by Fig. 2. 
Tig. 2. 
Should the profile of the work that is to be breached necessitate an 
angle of descent which cannot be attained at that distance which, 
as regards probability of hitting and final velocity, is such as gives 
promise of sufficient effect, we must have recourse to overcharged 
