[281] For example, when sweeping the crest of the parapet of a line of trenches, or the edge of a wood, both hands move the gun slowly and evenly from side to side. When searching an area in the direction of depth and obliquely (progressive fire with sweeping), the left hand gives the gun the proper horizontal direction, while the right manipulates the slow motion elevating gear. When firing on rapidly moving targets--for example skirmish lines advancing by rushes--or targets advancing over rolling country, both the traversing and elevating movements may be unclamped. The rapidity with which the gun is moved, when sweeping or searching, depends upon the range and the kind of target on which the fire is directed. As a rule, the piece is moved slowly and steadily. The accuracy of the fire is impaired when the gun is moved too rapidly. When the fire is well observed, it might be advantageous, in exceptional cases, when firing against either stationary or moving targets, to direct the gun, without aiming, after the bullets have been seen to hit their mark, by properly manipulating the elevating and traversing apparatus while the firing is in progress. The ballistic properties of the gun are the same as those of the infantry rifle.[282] [282] The destructive power of the projectiles fired from a machine gun, as they strike within a small space, is, of course, much greater than that of the scattered projectiles of a body of infantry. Trees having a circumference of 30 cm. are felled by machine gun fire in about 15 seconds at a range of 450 m. In the machine gun an important factor in the dispersion of infantry fire--flinching and errors in aiming--is eliminated, while the heating of its barrel and the vibrations of its carriage in continuous fire do not produce a corresponding increase in dispersion. On this account the cone of dispersion of the machine gun is more compact than that of the infantry rifle and its accuracy at long ranges is therefore considerably greater than that of the latter.[283] Firing tests indicate that the accuracy of machine gun fire diminishes only very slightly with increasing range, provided the appropriate elevation is used.[284] [283] According to Austrian experiments the depth of the beaten zone of a machine gun is only ¹⁄₃ to ¹⁄₂ that produced by the fire of a platoon of infantry. [284] The following average results were obtained in experiments made at the Musketry School, while firing on infantry targets advancing alternately at quick and double time: At ranges from 2000-1600 m. 1.72% hits „ „ „ 1500-1200 m. 2.53% „ Firing against disappearing head targets placed at intervals of 0.60-0.70 m.: At 600-800 m. 1.89% hits „ 800-1100 m. 1.69% „