Aiming triangle

The aiming triangle consisted of a wooden triangle and an alidade, with a 90° field of observation. The base of the triangle was equipped with a metal band, divided into degrees and tenths of a degree, that is, each degree is divided into 10 parts, each of which corresponds to 6’. However, each fraction could be easily divided by eye into two halves, so that the observer could report his observations to the nearest 1/20°, i.e. 3’. The divisions that correspond to a whole degree were not numbered to be numbered by the observer himself when orienting the triangle during the indirect fire, while the half degrees were marked with the number 5.
The alidade had two dioptres: the objective, had a window with crossed threads, three vertical ones of which divided it into four equal intervals, 1 cm long; the eyepiece, had an oblong notch with a scale in millimetres, along which a cursor moved. The alidade with its flat side rested on the triangle and moved around an axis fixed at 56 cm from the base of the triangle on the perpendicular raised from the centre of the base. The distance between the objective and the eyepiece was 1 m, so each interval between the threads of the objective was equal to 1/100 of the length of the alidade between the dioptres.
For use, the aiming triangle was placed on any support and fixed with pegs or iron plates. Use of the aiming triangle The aiming triangle could be used to determine: a) The deflection of the projectile in the distance with two aiming triangles (bilateral observation), the observers proceeded as with the Moller device. However, both alidades had to be positioned at the same degree on the scale and had to be directed to the central threads of the objective dioptre at the same point on the target. To determine the deflection, the observers should quickly direct their alidades to the point where the projectile fell, moving them to the left or right. The observers could do the same thing without moving their alidades, but, to obtain greater accuracy, they had to divide by eye each of the intervals between the threads of the objective dioptre into 10 equal parts, so that each part, which is 1/1000 of the distance between the two dioptres, gave an accuracy of 3’. In this case, the central thread of the objective dioptre was marked 0 and the divisions increased to the right and left up to 20. b) The lateral deflection of the projectile, the alidade was directed so that the central thread of the objective dioptre coincided with the point with respect to which the lateral deflection would be evaluated. At the moment of the shot, the observer, without moving the alidade, estimated by eye against which division the projectile was directed and, knowing that the value of a division was 1/1000 of the distance from the target, he could also determine the deflection in meters. c) To determine the height of the dispersion of the shrapnel, the alidade was directed so that the aiming line, passing through the hole of the cursor positioned at any point of the dioptre, and through the transverse thread of the objective dioptre, pointed at the target. Then, knowing that by lowering the cursor by 1 mm, the measuring line would rise above the target by 1/1000 of the distance to it, the observer could determine how many millimetres the cursor should be lowered so that the aiming line would pass a certain number of meters above the target. At the moment of the shot, the observer determined whether the dispersion had fallen on the predetermined line or how many meters above or below it. The height of the dispersion thus determined, although not completely accurate, was still considered sufficient for practical use. d) To determine the position of the target with indirect fire. The aiming triangle was also be used at the target training observation point to measure long and short falls, as well as the burst interval with time fire.
SOURCE : Ръководство за занятията въкреспостата артилерия. Част IV. Подготовка и служба на наблюдателите, Sofia 1902, pp. 34-39.

Aiming triangle	Alidade