THE ROYAL ARTILLERY INSTITUTION. 
75 
For enfilading particular faces at a distance of 800 to 1500 (or, excep¬ 
tionally, 1800) metres, the sliort 15 c.m. gun is the most suitable piece. 
In the case of badly defiladed lines, where an angle of descent of 10° to 
15° is sufficient, the 12 c.m. B.L. gun can be used, if the work to be 
done consists in disabling the guns between the traverses and clearing 
out the troops on duty. For cutting away the crests and destroying 
the traverses this calibre is not to be recommended. When using the 
short 15 c.m. against well defiladed lines the charge may be reduced 
to 0'5 k.g'. Nevertheless, charges under 0‘7 k.g. do not give good 
results, because they are not sufficient to ensure either regularity of 
flight or proper action of the fuzes. The short 15 c.m. gun is to be 
preferred for cutting down and destroying the earthen traverses. The 
experiments regarding the suitability of the rifled mortar, fired at a 
low angle, to effect the same object, are not yet concluded. 
The long 15 c.m. gun can be used in curved breaching fire if low 
angles of descent are sufficient. If higher ones are necessitated by the 
nature of the profiles the short 15 c.m. is the only gun that can be 
used. In arming captured works for breaching purposes the 12 c.m. 
B.L. gun of 1861 must, m general, be employed, in the absence of a 
more handy and powerful piece. 
The distances to be taken in the case of the older fortresses lie between 
750 and 1200 metres. At the latter limit the breaching will be less 
systematic and give smaller results. With escarp walls which, in con¬ 
formance with modern principles, are protected by earthworks, con¬ 
siderably higher angles of descent (20° to 25°) are necessary; and these 
are only attainable in conjunction with the requisite final velocity at 
much greater distances, when, in consequence of the large vertical spread, 
no great result is to be expected from either breaching or demolition fire. 
In such cases we must try to reduce the height of the covering parapet 
by means of mines, or by firing at it, and thus, by altering the profile 
conditions, enable ourselves, to employ curved demolition fire against 
the scarp at a shorter range and with a sufficient final velocity. 
In order to determine the limits to which the charge may be reduced 
it must be borne in mind that, according to all experience, when em¬ 
ploying the 15 c.m. gun against moderately strong masonry, a final 
velocity of at least 160 metres (with the 12 c.m. of 220 metres) is 
necessary, in order to produce a breach without undue expenditure of 
ammunition. This (in the case of the short 15 c.m.) indicates a charge 
of 0*75 k.g. as the smallest that can be used for breaching or demoli¬ 
tion fire against a scarp at all capable of resistance. 
In the case of a larger calibre—say the 21 c.m.—the object can be 
attained with a considerably less final velocity, and a high angle of 
descent is consequently obtainable at a shorter distance. As, however, 
the vertical spread increases with this angle, and as the heavier the 
calibre the greater is the difficulty of getting the guns into position, we 
shall not know, till after the completion of the contemplated experi¬ 
ments, how far the employment of the heavier calibres is advantageous. 
At the experiments at Graudenz in 1873, when a 21 c.m. gun and 
a short 15 c.m. were employed under similar conditions (final velocity 
208 metres, angle of descent 6° 45', obliquity of the line of fire 57°), 
