(18) v(di/dt) = g cos i,
where di denotes the infinitesimal decrement of i in the infinitesimal increment of time dt.
In a problem of direct fire, where the trajectory is flat enough for cos i to be undistinguishable from unity, equation (16) becomes
(19) v(di/dt) = g, or di/dt = g/v;
so that we can put
(20) Δi/Δt = g/v
if v denotes the mean velocity during the small finite interval of time Δt, during which the direction of motion of the shot changes through Δi radians.
If the inclination or change of inclination in degrees is denoted by δ or Δδ,
(21) δ/180 = i/π, so that