c
${\rm{T}} = 2\pi \sqrt {\frac{l}{{\rm{g}}}} $
when immerge in liquid
$\mathrm{mg}^{\prime}=\mathrm{mg}-\mathrm{F}_{\mathrm{up}}$
${{\rm{g}}^\prime } = {\rm{g}} - \frac{{{\rm{v}}{{\rm{d}}_l}{\rm{g}}}}{{{\rm{v}}{{\rm{d}}_0}}}$
$g^{\prime}=g(1-\rho)$
${{\rm{T}}^\prime } = 2\pi \sqrt {\frac{l}{{{\rm{g}}(1 - \rho )}}} $
$\frac{{T'}}{T} = \frac{1}{{\sqrt {1 - \rho } }} \Rightarrow {T^\prime } = \frac{T}{{\sqrt {1 - \rho } }}$