THE ROYAL ARTILLERY INSTITUTION. 
377 
Direct fire is when the projectile without grazing strikes an upright 
object not under cover. Indirect fire is when the object fired at is under 
cover, or when the projectile strikes the object after having grazed. 
To the above must be added, cross-fire when the lines of fire of several 
guns aimed at the same object cross each other; and diverging fire, when a 
number of guns in one battery fire at different objects. 
The use of the various natures of fire will be spoken of under the head of 
mountain guns in action. 
( b ) Extreme ranges, and probability of a hit, 
Extreme ranges. 
26. By extreme ranges is meant the furthest limit of the trajectories of 
the different projectiles: they are as follows :— 
Tor common shell, low-angle fire. 2490 yards. 
n a a high a . 1490 a 
„ shrapnel „ low „ 1250 „ 
„ case shot. „ „ 250 „ 
Circumstances upon which the probability of a hit depends . 
27. A given object is struck by a projectile, when the trajectory of the 
projectile intersects the object. In order then that each projectile fired 
should hit the object at a certain range, all the trajectories must be identical, 
or only slightly differing, according to the size of the object fired at. We 
know from experience that the trajectories vary under apparently similar 
conditions; seeing that they are influenced by elements whose nature and 
amount of effect can neither be accurately ascertained or taken into account. 
We cannot therefore with the best made guns and under the most favourable 
circumstances reckon on absolute certainty, but merely on a certain proba¬ 
bility of a hit. 
In par. 15 we have seen, as regards smooth-bore guns, that the effect of 
the windage, the possible rotation of the spherical projectiles on different 
axes and in different directions as well as the variation in the angles of 
departure, are prejudicial to the regularity of the trajectories; further, that 
with rifled guns these circumstances prejudicial to the probability of a hit 
are in a great measure got rid of by the steady bearing in the grooves 
and regular rotation of the projectile. 
Besides the above-mentioned there are yet other elements which affect the 
trajectory of the projectile and which are prejudicial to the probability of 
hit of rifled and smooth-bore guns alike; they are as follows :— 
(l) The ammunition as manufactured is not precisely identical; the powder is 
affected by atmospheric influences and by jolting in transport. The amount of 
force evolved varies with the quality of the powder, resulting in variation in 
initial velocity, and range. The force of the powder is to a certain extent dependent 
on the temperature of the gun when fired, for it is a well-ascertained fact that with 
a gun heated by long firing greater ranges are obtained than with a gun which is 
cold on opening fire. The resistance sustained by the projectiles in their flight 
through the air is often modified by the temperature and density of the atmo¬ 
sphere ; it thus happens, that on a warm day with a low barometer greater ranges 
are obtained than on a cold day with a high barometer. 
