THE ROYAL ARTILLERY INSTITUTION. 
77 
the height of the crest of the cover, in order that none of the projectiles 
may touch the latter. We must therefore add to the height of the 
crest at least three times the 50 per cent, vertical spread for the distance 
x v The vertical spread depends, however, on the charge, which latter 
must in the first instance be settled. In order, therefore, to be per¬ 
fectly safe, and to avoid all chance of risk, we take three times the 
50 per cent, vertical spread for the distance x x for the smallest allow¬ 
able charge. 
Lastly, in calculating the values of tan a and tan n 3 , the position of 
the lowest point of impact—in other words the value of y 2 —must be 
determined. 
In curved defensive fire the objects of fire which have to be taken 
into account will never be very high. We must take the lowest point 
of impact at the foot of the object, commence firing with the charge 
and elevation calculated accordingly, and afterwards, according to the 
observed effect, raise the point of impact by a corresponding increase 
of the elevation. 
In curved dismounting fire we should take the lowest point Of impact at 
the outer aperture of the embrasure; or, if there are no embrasures,- at 
the outer crest of the parapet, and proceed in the manner just indicated. 
If the cover is very close to the gun, or very high, the value of tan n l3 
and therefore also the angle of elevation, will be very large; and, at 
short distances, much too small a charge will be necessitated and con¬ 
siderable vertical spread entailed. On this account the employment of 
curved defensive fire is very limited, and is in general only possible at 
the longer ranges. 
In curved dismounting fire this inconvenience can be obviated by 
placing the gun at a greater distance behind the cover, so as to make 
the ground angle n 1} and consequently p 2 , as small as possible, and 
thus admit of the employment of large charges. In any case the 
angle n 1 must be smaller than the angle of elevation corresponding 
to the smallest allowable charge at the given range. 
The following example will show what further steps have to be taken 
to find the charge by means of the practice tables. 
Ex.—-A short 15 c.m. B.L. gun has to fire over a covering parapet 
the crest of which is 5 metres above the muzzle and 60 metres in hori¬ 
zontal distance from the gun, at a siege battery 758 metres from the gun, 
the foot of the battery being 1‘4 metres below the level of the muzzle. 
From the practice table we find that for 60 metres, with the smallest 
charge (0*5 k.g.), the 50 per cent, vertical spread is 0*066 metres; we 
shall therefore have 
y 1= = 5 + 3 X 0-066 = 5*198, 
tan ^ = - 1 ' 4 = 0'0Sfi63 + 0-00704 = 0-09367,* 
60 758 
0 2 = 5° 21'; 
* If the angles n 2 and a can be ascertained by means of an instrument for measuring 
angles, we have 0 2 = n x + a without further calculation. If n x and a, as also the horizontal 
distances m and x 2 — x x are known, a simple calculation gives the elevation or depression of the 
point of impact with reference to the level of the muzzle, whence it is easy to calculate n 2 . 
