THE ROYAL ARTILLERY INSTITUTION. 
67 
of impact at half the height of the strikeable portion—that is, at 
3*65 
—— = 1-825 metres higher. The intended point of impact will there- 
2 
fore be 1*825 — 1*167 = 0*658 metres above the level of the gun's 
muzzle, and we shall have 
tan n 0 — ° ^ = 0-000?. 
945 
Consequently n 0 — 3', and, finally, the correct angle of elevation will 
be 10° 8'. 
Ex. 4.—A line 80 metres in length, and without traverses, has to be 
enfiladed by the short 15 c.m. B.L. gun. 
The crest of the protecting parapqt is 3*5 metres above the level of 
the terreplein, and 2000 metres from the battery. By means of the 
quadrant the ground angle of this crest is found to be + 35'. In 
the practice table we find that the angles of descent at 2000 metres 
are— 
with 1*5 k.g. = 12° 20'. 
„ 1*4 „ = 13° 23'. 
„ 1-3 „ = 14° 36'. 
„ 1-2 . = 16° 5'. 
The distances behind the crest, corresponding to these angles of 
descent, at which the mean trajectory strikes the terreplein, can be 
obtained with sufficient exactness by the formula 
We shall have 
_ h 
tan 
with 1*5 k.g. z — 16 metres. 
n 1 * 4 // z = 14 ‘ 7 n 
n 1*3 n z = 13*4 a 
n 1*2 a z = 12T a 
If we decide to employ the charge of 1*2 k.g. we shall have a 
strikeable length of terreplein of 80 — 12 = 68 metres. 
The angle of elevation taken from the practice tables (reckoned from 
the straight line joining the centre of the gun's muzzle and the crest of 
the parapet) is 14° 36'. In order that the mean trajectory may clear 
the crest the ground angle must be added to this angle. We have, 
therefore, <j> = 14° 36' + 35' = 15° 11'. 
Finally, in order to bring the mean point of impact to the centre of 
the strikeable portion of the terreplein, the trajectory must be raised 
sufficiently to increase the range by 
68 
2 
-, or 34 metres ; which corresponds 
to an increase of the elevation by 18'. We have consequently, for the 
first trial shot, an angle of elevation of 15° 27'., or in round num¬ 
bers 15^°. 
Since the practice tables give a length of 25*15 metres for 50 per 
