=4. When combined Sights are to be Used.=--(i) Under service conditions it may be assumed that _even if the range is measured with the range-finder, the probability of error in ranging and judging atmospheric influences, known as the error of the day, is such that, at distances beyond 1,000 yards, collective fire concentrated on any target with one sighting will probably be ineffective_. (ii) To give a satisfactory degree of assurance of fire effect, it is advisable in such cases, _unless sighting can be corrected by observation of results_, to distribute fire in depth by using two elevations differing by 100 yards. One of these elevations would be 50 yards _over_ and the other 50 yards _under_ the sighting believed to be correct. (iii) There will thus be two cones of fire and two beaten zones overlapping between the nucleus of each cone of fire, and an even distribution of fire will be obtained over a zone about 150 yards deep (Fig. 7, D). Thus, if one sighting had been used, as shown in Fig. 7, B, the fire would have been ineffective, though the grouping of bullets in the zone of effective fire would have been more dense, because the error in sighting would have had the result described in para. 3 above. By using combined sights fire becomes effective, because though the grouping of bullets is less dense fire is _distributed in depth_ so as to avoid the result described in para. 3. (iv) _Ordinarily fire should be closely concentrated with a view to observation of results, but if observation has failed, or if the situation calls for immediate application of effective fire, combined sights should be used at ranges beyond 1,000 yards._ Combined sights may sometimes be used to increase assurance of effect when the enemy’s position is ill-defined, but should never be used if observation of results can be obtained. _Combined sights should not be employed by bodies of less than two platoons._ Section =14=.--The Relation of Ground to Fire Effect. =1.= So important is the influence exercised on fire effect by the shape of the ground in relation to the grouping of bullets, that it is essential for all officers and non-commissioned officers to understand thoroughly how the probability of fire effect is increased or diminished by the inclination of ground with reference to the trajectory. =2.= In attack, such knowledge will assist them in _adopting formations and directing the fire of their men to the best advantage_. In defence, it will aid them _to select the best positions for fire action_, and will enable them to take steps to minimize the inherent disadvantages of these positions. In the following examples only _plane surfaces, without undulations or accidents, are considered_. =3. Level Ground.=--As already stated, on level ground the zone beaten by collective rifle fire varies considerably with the range. The extent of this zone is further influenced by the _inclination of the ground to the line of sight_.