DIFFICULTIES OF AN ARTILLERYMAN.

(The following article is based on lectures delivered at the Saumur Artillery School by Captain Gaston Dreyfus and Lieutenant André Sorne, of the French Field Artillery, and instruction given at the Heavy Artillery School at Angers by Captain Maurice Barret of the French Heavy Artillery. The school at Saumur is the French Military Academy, one of their three schools corresponding to West Point, and during the war American enlisted men and officers were assigned there for instruction, together with artillerymen from the Polish armies. The French having trained plenty of officers in times of peace, reduced their own classes during the war almost to the vanishing point, only about fifty French students being in attendance during the summer of 1918. The instruction at the school was confined to 75 mm. guns and 155 mm. howitzers, with some few lectures on material, etc., of the 155 G.P.F. For this reason, officers destined for service in the heavy artillery went from this school to the Heavy Artillery School at Angers, a short distance down the Loire from Saumur. There they received instruction in more accurate surveying methods than those used by the Field Artillery, firing data for heavy guns, orientation by observations of the sun and stars, etc.)

Up to 1914 the artilleryman used either the rather rough-and-ready methods of the light field artillery, or the careful, extremely accurate methods of the coast and fortress artillery, the latter requiring months of painstaking preparation to be effective and therefore being inapplicable to mobile heavy artillery in the field. The unexpected use of heavy artillery in the field, as an integral part of the mobile armies, opened an almost endless path of difficulties to the heavy artilleryman. Position or trench warfare, constantly necessitating more and more accurate work by the light artillery, brought much the same troubles to that branch of the service.

Before discussing the solution of some of these difficulties, let us enumerate some of the factors which necessitate correction, in order to obtain accurate fire. The weight of the projectile cannot always be uniform, as is possible in time of peace: under the stress of war time manufacture a very material latitude in this respect must be allowed the manufacturer. This has two opposing effects, first, since with a given charge the momentum of the projectile will be constant, it follows that a heavy projectile will have a lower velocity than a lighter one, and therefore will not go so far. On the other hand, the heavy projectile will experience less resistance from the

air, and this will tend to make it go farther. At short ranges the first effect predominates, at long ranges the other, and, both must be taken into account; the resultant of the two effects, calculated for the varying weights, is included in firing tables furnished to the artilleryman, from which he obtains the necessary correction without difficulty.

Obviously, the wind will affect both the range and the direction of a projectile. At first, correction for this factor was largely guess-work, but gradually meteorological sections along the Front made possible more careful observation. These sections reported to the batteries, at intervals, the direction and strength of the wind. This report, however, was not the actual wind which was blowing, but a so-called "ballistic wind", that is, a fictitious wind of a direction and strength such that if it were blowing at all altitudes, with uniform velocity, instead of the dozen or more varying winds actually blowing at various altitudes through which the projectile must pass, it would have the same effect on the projectile as the resultant effect of all the actual winds Knowing the direction of this wind and the direction of fire, it was then possible for the battery commander to resolve the force of the wind into two components, one acting either to increase or decrease the range, and the other affecting the direction only, and to correct for both.

A very important factor is the density of the air, which can readily be calculated and corrected for by observing the barometer and the thermometer. Theoretically the hygrometric state of the air should also be considered; practically, however, this was disregarded.

Theoretically, the temperature of the powder charge at the time of firing should be considered. Practically this was very difficult of measurement and was very nearly offset by errors in the opposite sense due to the enlargement of the powder chamber of the gun under the influence of the same increase of temperature which added to the force of the powder.

The overwhelming factor to be considered was the variation in the powder making up the propelling charges. This will be considered more at length in a moment.

No particular difficulties come up in connection with the other ordinary corrections, such as difference in altitude of gun and target, etc. An interesting series of corrections becomes necessary, however, at extreme ranges, and became very material in calculating firing data for guns such as those with which the Germans bombarded Paris; these are the ones due to the curvature and rotation of tthe earth. Obviously at ranges of 20,000 meters and more, the earth's curvature will cause a projectile to travel further than if it had been fired over a true plane. The correction for this factor for the German long-range gun is said by Captain Dreyfus to have amounted to over 1,500 meters. It is interesting to note that, while an elevation of 45 degrees theoretically gives maximum range, this gun was fired at an elevation of 53 degrees to give maximum range. At this elevation, the trajectory of the projectile was 45 degrees to the horizontal at the point where the projectile left the earth's atmosphere, so that a maximum range in approximately frictionless space was secured.

If a gun is fired toward the north, the projectile, having the east-and-west velocity due to the rotation at the latitude of the gun, will pass to a latitude which has a lower velocity, due to the earth's rotation, and will accordingly drift to the east. Conversely, a projectile fired south will tend to drift to the west. A combination of the earth's curvature and rotation comes into play in firing east and west. The projectile, of course, has the east-and-west velocity of the place from which it was fired, acting in a tangent to the earth's surface through that point. Toward the end of its trajectory, it still has that velocity, (disregarding velocities due to other causes, such as that received on firing the gun), still acting in a direction parallel to that tangent; but the surface of the earth below it has a velocity which it acting in the direction of the tangent at that second place, which is at an angle to the tangent through the place where the gun was fired. Therefore that part of the velocity possessed by the projectile which is due to the earth's rotation, must be resolved into two components, one of which is parallel to the surface of the earth below and which holds it in its proper position relative to that surface,

as if neither the earth nor the projectile had any rotational velocity. The other component, then, acts upwardly on a projectile fired east and downwardly on one fired west. Accordingly, everything else being equal, a projectile will carry further fired east than if fired west. These corrections due to curvature and rotation of the earth are usually of no more than academic interest, even to an artilleryman, since even when they must be considered at long ranges, they are incorporated in the firing tables.

In addition to the lateral component of the wind, the principal correction in direction is due to drift. There is still some argument as to why a projectile drifts to one side or the other, according to the direction of the rifling. It certainly is not for any of the reasons given to explain the curve of a pitched ball, for a ball is never thrown to rotate about an axis tangent to its trajectory, as is the case with a projectile, so there can be no question of rarifying the air on one side and making it more dense on the other, or any of the other explanations advanced by baseball fans. It is probably due to a gyroscopic couple, the resultant of adding vectorially the torque caused by the spin of the projectile itself, to the torque about a horizontal axis through the center of gravity, caused by the force of gravity trying to turn the projectile so that the heavier base will be below the center of gravity. This resultant would be a torque about a vertical axis in a direction dependent on the direction of the spin of the projectile itself. The correction is incorporated in the firing table, and need not be calculated in the field. There is no difficulty in applying it, therefore unless (as has been known to happen) it is added in with the wrong sign!

The difficulty in adjusting fire, after careful corrections for the foregoing items have been made, is due principally to the dispersion of the gun. A 75, which is a very accurate gun, with external conditions as nearly the same throughout the firing as possible, will spread its projectiles over a zone 76 meters long at a range of 1,000 meters, and this dispersion increases at greater ranges. The long-range German gun, presumably with the same setting for range and direction, deposited suc

cessive projectiles on opposite sides of the immense city of Paris. It is doubtful on account of this tremendous dispersion, whether this gun could have hit any smaller target than Paris, at any such range. Some time ago a correspondent of a popular scientific paper suggested that our long-range guns not needed by the army be taken over by the Postoffice Department for firing mail from one station to another. The correspondent suggested that the packages could be fired within one yard of the receiving station. If this suggestion is ever adopted, the War Risk Bureau will receive a new lease of life, caring for our brave mail clerks who have been lacerated by exploding packages of letters, which decidedly would not land within the prescribed yard unless by accident. Of course, more projectiles will fall near the center of this zone of dispersion than near the ends, but how is the observer to know which is the case for any particular burst which he sees? As a matter of fact, he does not know; his aim is not primarily to hit the target -although he may not be averse to doing so-but to so adjust the guns that as many bursts will be over the target as are short, thus giving a maximum probability of hits. In firing 75s and 155s on the range at Saumur twice a week for three months, only twice, much to my astonishment, did I succeed in knocking over a target (about two meters square), and I have always suspected that on one of those occasions the wind did the nefarious work, and the burst at the same instant was no more than a coincidence. That may be due to poor marksmanship, of course, but I spoiled as many targets as any of the other students.

Dispersion is due to many factors, slight differences in the laying of the gun for different shots, slight variations in the atmosphere from moment to moment, variations in the powder charges, differences in the loading of successive shots and many other causes. Changing crews on a gun, so that different men ram the projectiles into the bore, may make a difference of as much as 100 meters in the range of those projectiles, all other factors remaining the same. As a gun crew becomes tired, dispersion may increase fifty per cent or more over its original value with the fresh crew. Dispersion, of