Taking the limit N → ∞, h → 0 and assuming smoothness, one gets ∂ ∂ 2 u ( x , t ) ∂ ∂ t 2 = K L 2 M ∂ ∂ 2 u ( x , t ) ∂ ∂ x 2 , which is from the definition of a second derivative. KL / M is the square of the propagation speed in this particular case.