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alter at greatly varying rates there is usually some uncertainty as to the most suitable intervals in which they should be called out. If they are electrically transmitted, so as to move some part of the receiving instrument, the mechanism must be capable of moving in small steps of, say, one minute, in order to obtain the required accuracy, and at the same time it must be capable of moving quickly through a large angle when getting on to a target.

3. One-man Range-finders used for coast defences are constructed on the same general principles as those used in the field ; but, as their weight and size must not be limited to the same degree, many modifications to make them as accurate and convenient as possible are introduced. Their base lengths are usually between 9 ft. and 10 metres, but even longer bases have been considered. They are practically always of the coincidence type with both fields erect.

The Mark III Depression Range-finder (fig. 2) will be taken as a type of a depression instrument. In this range-finder the range- finding triangle is reproduced in the instrument on a small scale. Figure I shows diagrammatically how this is effected by means of

FIG. i.

two arms. AB represents the height of the axis of the telescope above the surface of the sea BC. Ab represents the distance between the pivot of the telescope arm Ac and the range arm be, and this to the scale of the instrument, represents the height AB. When the instru- ment is level, the arm cb is horizontal, and therefore parallel to the

FIG. 2. Depression Range-finder Mark III.

surface of the sea. cb is subdivided and graduated in equal divisions to the scale of the instrument. Ac, which has a telescope mounted on it, is pivoted at A and can be directed on to the water-line of a target at C. The triangles ABC and Abe are similar, and the length of be therefore represents to the scale of the instrument, which is 1,000 yd. to an in., the actual range BC. Similarly bd will represent the range BD of a target at D. If the instrument is required for use at any other height than AB, the telescope arm pivot A would be raised to a height above cb corresponding to the new height of the range-finder above the sea. In the foregoing, the surface of the sea has been considered as a plane surface, whereas it is really the surface of a sphere with its centre at the centre of the earth. Allow- ance for this curvature could be made by making the arm be the arc of a circle instead of a straight line. It is, however, found to be more convenient to curve the telescope arm Ac in the opposite direction, the effect being the same. Corrections for mean refrac- tion are made in the same way as those for the curvature of the earth, but in the opposite direction; the arm Ac being curved to allow for the combined effect of curvature and refraction.

The Small German Position-finder may be taken as a typical position-finder. It was used during the World War for coast artillery on low sites. Very much larger instruments based pn the same general principles were also used. Air communication was by telephone. Other nations often use automatic electrical transmission for moving parts of the receiving instrument when the transmitter telescope is traversed, and for actuating range and bearing dials on the gun mountings when the gun arm is traversed.

The Barr and Stroud Range-finder having a base-length of 30 ft. is typical of the one-man range-finder used for coast defences.

None of the range-finders or position-finders mentioned above is of any use if it cannot see the target ; the position of a -moving target can then only be ascertained by aerial observation.

Naval Range-finders, which must necessarily be of comparatively small size, are nearly always of the one-man type. They are gen- erally similar to those used in coast defences, but in order to obtain better illumination their optical parts are often made larger. Their mountings are designed to enable the range-finders being kept on the target in a sea way. They are nearly always of the coincidence type with both images erect; but some stereoscopic range-finders have been used by the Germans. It is thought by some that a range can be more rapidly taken from a ship having considerable motion with a stereoscopic range-finder than with a coincidence range-finder; as with the latter it is difficult to keep the separating line across the target. The coincidence range-finder will, however, give more accurate results. Range-finders with a base-length of 10 metres are the largest which can conveniently be used on board ship. Base lengths vary with the size of the ships on which the range-finders are employed and the uses for which they are intended ; one of less than a metre being employed for navigational purposes. Anti- aircraft range and height-finders used on ships must be provided with some form of pendulum to keep certain of the parts horizontal, as the mountings cannot be kept level. Certain small ranee-finders have been made which can measure the range to a ship if the height of its mast or funnel is known. This height is used as the base of the range-finding triangle, the angle of parallax being measured by the instrument and read off it as a range. Such a base is obviously longer than that of a range-finder which could be carried on a ship. This type, however, has not been generally adopted.

FOR AIR DEFENCE

Range-finders and height-finders used in connexion with anti-aircraft guns have a much more difficult task to perform than the range-finding instruments used in the field or in coast defences. In the latter cases the target, if it moves at all, will move in one plane only and its speed will not approach that of aircraft. As the result of experience, the range of an aerial target which may alter by several thousands of yards in a minute is not taken into account; but its height, which will not vary to the same extent, is used as the basis of laying 8 and is measured by a height-finder. The rate of burning of the fuze has also to be considered, as it will not be the same for a definite range if the target is at different heights.

When laying a gun on aircraft, the setting of the sight is therefore based on the height and angle of sight of the target and on the setting of the fuze. At the beginning of the World War, no height-finders were available, so existing one-man range-finders, e.g. the Barr and Stroud, were used, and long base height-finders which could be rapidly made were introduced.

FIG. 3.

One-man range-finders were usually of two metres base length, and coincidences were made on the target in the usual way. In order to convert the ranges into heights, a slide-rule attached to the range- finder was employed. One scale of this slide-rule was automatically set to the angle of sight of the target, by means of a cam, as the elevation of the range-finder was altered. The other scale was set to the range recorded by the range-finder, and the height of the target could then be read off.

Long base height-finders usually consist of two instruments at the ends of a base about a mile in length. Sighting planes in these instruments are kept laid on the target; the triangle formed by the intersection of a vertical plane with three planes, one of which is a horizontal plane passing through the base and the other two are extensions of the sighting planes, is mechanically solved; and the height of the point where the two planes of sight intersect (i.e. the height of the target above the base) is plotted at the same time.

The principle of this method is shown in fig. 3. AX and BY are two horizontal lines, parallel to each pther. The sighting planes of