Two forces $\bar { F } _ { 1 }$ and $\bar { F } _ { 2 }$ are acting on a body. One force has magnitude thrice that of the other force and the resultant of the two forces is equal to the force of larger magnitude. The angle between $\vec { F } _ { 1 }$ and $\vec { F } _ { 2 }$ is $\cos ^ { - 1 } \left( \frac { 1 } { n } \right)$. The value of $| n |$ is $\_\_\_\_$.