数列\(\{a _{n} \}\)中，\(a _{1} =1\)，\(a_{2}= \dfrac {1}{2}\)，\( \dfrac {2}{a_{n}}= \dfrac {1}{a_{n+1}}+ \dfrac {1}{a_{n-1}}(n\geqslant 2)\)，则\(\{a _{n} \boldsymbol{⋅}a _{n+1} \}\)的前\(n\)项和\(S _{n} =\)______．