It is evident that, if an aeroplane should be moving on a level with the apex of the trajectory considered and in the vertical plane containing the trajectory, with a velocity equal to the horizontal
component of the muzzle velocity, and should, at the apex, drop a projectile of the same weight, size and shape as that of the mortar, the projectile thus dropped would take the identically same path and strike with the same velocity as the mortar projectile.