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SIGHTS

Online Encyclopedia
Originally appearing in Volume V25, Page 62 of the 1911 Encyclopedia Britannica.
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SIGHTS, the name for mechanical appliances for directing the axis of the bore of a gun or other firearm on a point whose position relative to the target fired at is such that the projectile will strike the target. Gun Sights.—Until the 19th century the only means for sighting cannon was by the " line of metal "—a line scored along the top of the gun, which, owing to the greater thickness i into the navy; this was adopted by the army in 1846. In the case of metal at the breech than at' the muzzle, was not parallel to I of most guns it was used in conjunction with the dispart sight above the axis. Some allowance had to be made for the inclination I referred to. The tangent sight (see fig. 3) was graduated in degrees only. There were three patterns, one of brass and two of wood. As the tangent sight was placed in the line of metal, hence directly over the cascable, very little movement could be given to it, so that a second sight was required for long ranges. This was of wood; the third sight, also of wood, was for guns without a dispart patch, which consequently could not belayed at elevations below the dispart angle. Referring to fig. t it will be seen that in order to strike T the axis must be directed to G' at a height above T equal to TG, while the line of sight or line joining the notch of the tangent sight and apex of the dispart or foresight must be /~/~ directed on T. In fig. 4 the tangent sight has been raised from 0 to S, the line of fiGradonu ",a- L sight is SMT, and the axis produced is AG'. D is the dispart, M the muzzle sight, tangent OM is parallel to AG'. Now the height to sights. which the tangent sight has been raised in order to direct the axis on G' is evidently proportional to the tangent of the angle OMS = AXS. This angle is called the angle of elevation; OM is constant and is FIG. 3. called the sighting radius. If the dispart sight were EarlyTangent being used, the sighting radius would be OD, but, as Sight. at the range in fig. 4, the line of sight through D foals the metal of the gun, the muzzle sight M is used. The formula for length of scale is, length =sighting radius X tangent of the angle of elevation. In practice, tangent sights were graduated graphic-ally from large scale drawings. It will be seen from fig. 4 that if the gun and target are on the same horizontal plane the axis can be equally well directed by inclining it to the horizontal through the requisite number of degrees. This is called " quadrant elevation," and the proper inclination was given by means of the " gunner's quadrant," a quadrant and plumb bob, one leg being made long to rest in the bore, or by bringing lines scribed on the breech of the gun in line with a pointer on the carriage; these were called " quarter sights." Such were the sights in use with smooth-bore guns in the first half of the last century. Tangent sights were not much trusted at first. Captain Haultain, R.A., says in his description of testing sights (Occa- sional Papers, R.A. Institute, vol. i.) : " Raise the sight, and if it keeps in line with a plumb bob, it can be as confidently relied upon as the line of metal, if the trunnions are horizontal. If the scale is only slightly out of the perpendicular, a few taps of the hammer will modify any trifling error." The introduction of rifling necessitated an improvement in sights and an important modification in them. It was found that projectiles fired from a rifled gun deviated laterally from the line of sights for fire owing to the axial spin of the projectile, and that if the rifled spin were right-handed, as in the British service, the deviation was to the right. This deviation or derivation guns° is usually called drift (for further details see BALLISTICS). The amount of drift for each nature of gun at different ranges was determined by actual firing. To overcome drift the axis must be pointed to the left of the target, and the amount will increase with the range. In fig. 5(plan) at a range HT, if the axis were directed on T, drift would carry the shot to D, therefore the axis must be directed on a point D' such that D'T = DT. HFT is the line of sight without any allowance for drift, causing the projectile to fall at D. Now if the notch of the tan- gent sight be carried to H' in order to lay on T, the fore-sight, and , with it the axis, It will be moved to F', the line of fire will be HF'D', and the shot will strike T since D'T = DT. Left deflection has been put on; this could be done by noting the amount of deflection for each range and applying it by means of a sliding leaf carrying the notch, and it is so done in howitzers; in most guns, however, it is found more convenient and sufficiently accurate to apply it automatically by inclining the socket through which the tangent scale rises to the left, so that as the scale rises, i.e., as the range increases. the notch is carried more and more to the left, and an increasing of the line of metal to the axis" (Lloyd and Hadcock, p. 32). The line of metal does not come under the definition of sights given above. In the year 18os a proposal to use sights was sent to Lord Nelson for opinion, and elicited the following reply: " As to the plan for pointing a gun, truer than we do at present, if the person comes, I shall, of course, look at it, or be happy, if necessary, to use it; but I hope we shall be able, as usual, to get so close to our enemies that our shot cannot miss the object " (letter to Sir E. Berry, March 9, 1801). Three weeks later the fleet under Sir Hyde Parker and Nelson sailed through the Sound on its way to Copenhagen. In replying to the guns of Fort Elsinore no execution was done, as the long range made it impossible to lay the guns (Lloyd and Hadcock, P. 33). The necessity for sights follows directly on investigation of the forces acting on a projectile during flight. In a vacuum, the projectile acted on by the force of projection begins to fall under the action of gravity immediately it leaves the bore, and under the combined action of these two forces the path of the projectile is a parabola. It passes over equal spaces in equal times, but falls with an accelerating velocity 'according to the formula h=2gt2, where h is the height fallen through, g the force of gravity, and t the time of flight. From fig. I it will be seen that in three seconds the projectile would have fallen 144 ft. to G ; therefore to strike T the axis must be raised to a point 144 ft. vertically above G. This law holds good 3" ,r—^--- G FIG. I--Elevation. also in air for very low velocities, but, where the velocities are high, the retardation is great, the projectile takes longer to traverse each succeeding space, and consequently the time of flight for any range is longer; the axis must therefore be directed still higher above the point to be struck. The amount, however, still depends on the time of flight, as the retardation of the air to the falling velocity may be neglected in the case of flat trajectory guns. Owing to the conical shape of the early muzzle-loading guns, if one trunnion were higher than the other, the " line of metal " would no longer be in the same vertical plane as the axis; in consequence of this, if a gun with, say, one wheel higher than the other were layed by this line, the axis would point off the target to the side of the lower wheel. Further, the inclination of the line of metal to the axis gave the gun a fixed angle of elevation varying from I° in light guns to 24° in the heavier natures. To overcome this a " dispart sight " (D) was introduced (fig. 2) to bring the line of sight (A'DG') parallel to the axis (AG). D `'a AG is the axis of the bore, ab the dispart, A'DG' is parallel to AG. D is the dispart sight, S the tangent sight, A'DS the clearance angle. At greater elevations than this the muzzle notch is used; to align on the target at lesser angles the dispart sight is so used. Guns without dispart sights cannot be layed at elevations below the clearance angle. The earliest form of a hind or breech sight was fixed, but in the early part of the 19th century Colonel Thomas Blomefield proposed a movable or tangent sight. It was not, however, till 1829 that a tangent sight (designed by Major-General William Millar) was introduced c' 2' --G ----------------- 1 2 ^3 I amount of left deflection given—the amount can easily be determined thus: The height of tangent scale for any degree of elevation is given with sufficient accuracy by the rough rule for circular measure h = 3 X I zzoo where a is the angle of elevation in minutes, h the height of the tangent scale, and R the sighting radius; thus for 1°, h = 63600R = 6—0 • Now supposing the sight is inclined i ° to the left, which will move the notch from H to H' (see fig. 6); as before HH'=-, but in this case R=h=6o..HH'=6oRX6o' the resultant angle of deflection is HFH', and this can be determined by the same formula a=hXl2RooX3 but in this case h=HH'= R 6oX6o a R X 3600 = i', so that if the sight is inclined to the left 1 ° it will give i' deflection for every degree of elevation. By the same formula it can be shown that 1' deflection will alter the point of impact by I in. for every too yds. of range; thus the proper inclination to give a mean correction for drift can be determined. In the early R.B.L. guns this angle was 2° 16'. With rifled guns deflection was also found necessary to allow for effect of wind, difference of level of trunnions, movement of target, and for the purpose of altering the point of impact later-ally. This was arranged for by a movable leaf carrying the sighting V, worked by means of a mill-headed screw provided with a scale in degrees and fractions to the same radius as the elevation scale, and an arrow-head for reading. Other improvements were: the gun was sighted on each side, tangent scales dropping into sockets in a sighting ring on the breech, thus enabling a long scale for all ranges to be used, and the foresights screwing into holes or dropping into sockets in the trunnions, thus obviating the fouling of the line of sight, and the damage to which a fixed muzzle sight was liable. The tangent sight was graduated in yards as well as degrees and had also a fuze scale. The degree scale was subdivided to to' and a slow-motion screw at the head enabled differences of one minute to be given; a clamping screw and lever were provided (see fig. 7). Fore-sights varied in pattern. Some screwed in, others dropped into a socket and were secured b}/a bayonet joint. Two main shapes were adopted for the apex—the acorn and the hogsback. Instruction in the use of sights was based on the principle of securing uniformity in laying; for this reason fine sighting was discountenanced and r_-. _-__, laying by full sight enjoined. " The ~/\/\/ centre of the line joining the two highest points of the notch of the fore-FIG. 8.—Laying by Full tangent sight, the point of the fSight. sight and the target must be in line " (Field Artillery Training, 1902) (see fig. 8). Since the early days of rifled guns tangent sights have been improved in details, but the principles remain the same. Except for some minor differences the tangent sights were the same for all natures of guns, and for all services, but the development of the modern sight has followed different lines according to the nature and use of the gun, and must be treated under separate heads. Sights for Mobile Artillery. With the exception of the addition of a pin-hole to the tangent sight and cross wires to the fore-sight, and of minor improvements, and of the introduction of French's crossbar sight and the reciprocating sight, of which later, no great advance was made until the introduction of Scott's telescopic sight. This sight (see Plate, fig. 9) consists of a telescope mounted in a steel frame, provided with longitudinal trunnions fitting into V's in the gun. These V's are so arranged that the axis of the sight frame is always parallel to that of the gun. By means of a cross-level the frame can be so adjusted that the cross axis on which the tele- scope is mounted is always truly horizontal. Major L. K. Scott, R.E., thus described how he was led to think of the sight : " I had read in the Daily News an account of some experimental firing carried out by H.M.S. ' Hotspur ' against the turret of H.M.S. ' Glatton,' At a range of 200 yds. on a perfectly calm day the ' Hotspur ' fired several rounds at the ' Glatton 's ' turret and missed it." Major Scott attri• buted this to tilt in the sights due to want of level of mounting (R.A.I. Proceedings, vol. xiii.). Tilt of sights in field guns owing to the sinking of one wheel had long been recognized as a sourceof error, and allowed for by a rule-of-thumb correction, depending on the fact that the track of the wheels of British field artillery gun-carriages is 60", so that, for every inch one wheel is lower than the other, the whole system is turned through one degree- a=hX1 X3=hX6o=6o' or I°, as his I inch. Referring to the calculations given above, this is equivalent to 1' deflection for every degree of elevation, which amount had to be given towards the higher wheel. This complication is eliminated in Scott's sight by simply levelling the cross axis of the telescope. Other advantages are those common to all telescopic sights. Personal error is to a great extent eliminated, power of vision extended, the sight is self-contained, there is no fore-sight, a fine pointer in the telescope being aligned on the target. It can be equally well used for direct or indirect, forward or back laying. A micrometer drum reads to 2', while the vernier reads to single minutes so that very fine adjustments can be made. Disadvantages of earlier patterns were, the telescope was inverting, the drum was not graduated in yards, and drift not allowed for. These defects were all overcome in later patterns and an Scott's important addition made, viz. means of measuring the si ht. angle of sight. In speaking of quadrant elevation a brief g reference was made to the necessity for making an allowance for difference of level of gun and target. Figs. lo to 13 explain this more fully, and show that for indirect laying the angle of sight must be Target
End of Article: SIGHTS
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