reille: anti-aircraft firing 471 



combustion of the fuse is delayed by the dilution of the air, and 

 the burst occurs not on the isochrone curve but further along, 

 on another curve which deviates from the former in proportion 

 to the time during which the projectile, before bursting, has been 

 subjected to a lower barometric pressure. This other curve, de- 

 pending upon a geometric length of combustible tube and not 

 upon a chronometric length of flight, is sometimes called the 

 "isopyre curve" (fig. 3). 



Since the beginning of the war the empirical outHning of the 

 isopyrical curves and of the trajectories for high altitudes has 

 been the object of very careful work in which thousands and 

 thousands of projectiles have been used. A standard diagram 

 has been estabUshed (Puteau-Arnouville, 1915-16), which, used 

 either in its original form or more or less ingeniously trans- 

 formed, constitutes the essential instrument of all the methods 

 of firing against aerial targets. 



IV 



Anti-aircraft firing does not consist merely in firing at an 

 aerial target, but in firing at an aerial target in motion. More- 

 over, this target moves with a speed which cannot be regarded 

 as negUgible with reference to the speed of the projectile de- 

 signed to strike it. 



With an average wind, an "observation machine" attains a 

 speed of 35 meters (38.15 yards) per second. At ordinary firing 

 ranges the time of flight of the projectile amounts to twenty 

 seconds. It follows that, under normal conditions, the distance 

 covered by the target, between the moment at which the pro- 

 jectile designed to strike it is fired and the moment at which it 

 bursts is about 700 meters. 



What will be its course? How can the gunner locate in ad- 

 vance the position in space where the target anjd the projectile 

 wiU meet after both have followed their respective trajections 

 for the same length of time? This is the problem sometimes 

 caUed, especially by the British, the problem of prediction. 



The target being an animated one and having, as one might 

 say, its own will-power, it is a priori obvious that no absolute 

 and definite solution can be applied to the problem. During 



