The requirements which an indirect method of aiming must fulfill are,

I. The position of the battery must not be prematurely disclosed to the enemy.

2. It must be independent of the range and of the kind of

cover.

3. It must permit a quick change of target and also the shooting at objects in motion.

4. The operation of the apparatus devised must be simple and in accordance with conditions of field service.

This last requirement is the most difficult of all to secure.

METHOD OF CAPTAIN ROKNIC.

In the accompanying figure let Z be the target, Wa wood as a mask, and G the position of a gun to represent the line of the batteries.

Construct the right-angled triangle A GZ, and lay off on the cathetus AG a definite distance AB, so that the target can be seen from B. Erect at B, the perpendicular Ba and at a the perpendicular ab; we thus have two. similar triangles ab B and A B Z, in which A B and B a are known and ab can be measured since the target is visible from B; then ab: AB :: BA : aB AB

AZ from which AZ =

ab

The points G and determine the direct line of sight of the gun. Based upon this geometrical method, Captain Roknic, of the Fourth Corps Artillery Regiment, has constructed an apparatus, which not only makes possible the establishment of two perpendiculars by means of two sights standing perpendicular to one another, but also enables us to measure directly the catheti BA and Ga' and a b and a' b'.

USE OF THE APPARATUS.

The apparatus is set up at A and the target Z be sighted over the long sight arms V and K.

B

OK K

Fig. 2.

While the sighting is being done, two men stretch a cord 75 m long from A to B, the man at B being directed on the perpendicular V' K" (over the short sight arms) the apparatus will then be transferred to B after setting up a sighting stake at A. At B the mmarm will be revolved into place, the line of sight of the short arm, K" V', will be directed on A; then sighting Z over V and pushing the sight K' along the mm scale

until it falls in the line VK' Z we have only to read off the number of mm, which gives the length of the cathetus a b of the preceding figure.

Next take the apparatus from B to G and set it up exactly as at B. As previously at B, move the sight K' to nxab mm A

[n=4G.ab] which gives for the guns the line of azimuth

A B

sight in the plane of fire. The transfer of this azimuth to the neighboring guns is accomplished by means of a cord stretched. over the sight of the gun and the direction stake. For the concentration of fire the interval of these direction stakes must be a definite proportion of the interval of the guns.

The apparatus can also be used for the approximation of ranges; by means of the similarity of ABZ and a bB (see figure page 174) there results AZ:AB::aB:ab

The apparatus was tested in the year 1891 on the proving grounds near Orkeny and gave, on the whole, satisfactory results. The instrument possesses however little stability and must be cautiously handled to prevent (spontaneous) derangement. The method is complicated and not suitable for the conditions of field service.

METHOD OF CAPTAIN von BRILLI.

In the summer of 1890, the then Captain von Brilli made in

the camp of Bruck his first published experiments with reference to this subject. He based these experiments upon an angle mirror which he replaced the next year by an angle prism. Brilli's indirect aiming depends upon the establishment of the direction of shooting and the base line perpendicular thereto; further, in making the lines of sight of the guns perpendicular to the base line, that is, making them parallel to the direction of shooting.

This project of Captain von Brilli's was, however, not accepted at that time by the technical military committee, above referred to, and was first brought to a practical trial in the year 1894, somewhat modified, by leaving out the auxiliary cross bars and the use of an aiming contrivance on the ends of the axle of the wheels.

NOTE. The following description is taken from the brochure of Captain von Brilli.

For carrying out the indirect method of aiming there are required for every gun one angle prism with staff, for each battery one cross bar and and one angle prism with staff, one sight staff, and two long stakes.

The use of the angle prism to mark out the perpendiculars depends upon the fact, that a ray falling upon the side at an angle greater than 41° 48′ will not be transmitted, but reflected (movable image-single or total reflection); by double reflection the image is constant and makes with the ray of light an angle of 90°. These images will be made use of in marking out the perpendiculars. On the line v Z let it be required to erect a perpendicular VR at the point v. Hold the prism in v so that the image from R falls on the cathetus surface AB, at the same time keeping the eye upon Z by looking away over the prism. Bring the mirrored image of R in the prism to coincide exactly with

800

Ocular Ebene

Fig. 4.

Objectio Ebene

the point Z seen directly. To establish this line Rv bring into it the stake R', marked so as to be recognized when the super-imposed images of the stakes appear in the prism.

The cross-prism is used to mark upon the cover lying before an intermediate point (stand-point of the observer) on the line connecting the target and the battery which are not reciprocally

visible. This point indicates to the battery the line of fire. The cross prism consists of two three-sided prisms, the cross-sections being isosceles right angled triangles, of which the hypothenuses are perpendicular to each other and lie in planes one above the other. In the figure, let a point B be chosen upon the cover D between the target Z and the position of the guns G (or the point B' between an auxiliary aiming point H and a high stake R situated at the side and a little in rear of the battery). Take the instrument and stand on the point (E or E'), as the case may be, near the line GZ or RH and hold it so that the ocular plane is turned toward the eye, the object planes towards G and Z (or R and H). The images must now be brought to coincide.

The gauging-rod serves to establish an auxiliary point in the terrain just as far to the right or left of the target as the observer himself is to the right or left of the line of fire of the ranging gun. It is about 10 cm. long, made of celluloid and can be made fast with a cord to the highest button of the coat. The length of the cord is so determined that when held taut, the gauging rod will be 50 cm. in front of the eye. There are two scales on the gauging-rod. Scale No. 1 serves to determine the distance in 100 paces of auxiliary points which are situated right or left of the middle of the target. The length of the scale from zero in mm., 500 carried out for the various distances, amounts to wherein

n

n = 100 paces. To use this instrument the zero of the rod when held horizontally will be directed on the middle of the target. The second scale is used for measuring the angles of slopes; each division corresponds to two divisions of the elevating arc, and amounts for the distance of 50 cm. from the eye to 1.33 mm. This scale is used also to estimate the bursting height when shooting with shrapnel.

The stakes are two and one-half m. long, 3 to 4 cm. in diameter and are provided with an iron shoe together with a cross bar. In order that they may be more sharply defined at great distances. one of the stakes is painted white, and the other red.