\begin{table}%t2
%\centering
\par
\caption{\label{tab:pdot}Contributions to the period derivative.}
\begin{tabular}{l@{\hspace{0.30cm}}
c@{\hspace{0.15cm}} c@{\hspace{0.15cm}}
c@{\hspace{0.15cm}} c@{\hspace{0.15cm}}}
\hline\hline \noalign{\smallskip}
PSR &  $\dot{P}_{\rm obs}$ & $\dot{P}_{\rm Shk,max}$ & $\dot{P}_{\rm Gal,\perp}$ & $\dot{P}_{\rm Gal,\parallel}$ \\
\hline  
J1721$-$2457 & $5.55 \times 10^{-21}$ & $2.5\times 10^{-21}$ & $4.0 \times 10^{-23}$ & $4.4 \times 10^{-22}$ \\
J1745$-$0952 & $9.23 \times 10^{-20}$ & $4.9\times 10^{-20}$ & $4.5 \times 10^{-22}$ & $2.8 \times 10^{-21}$ \\
J1810$-$2005 & $1.47 \times 10^{-19}$ & $5.4\times 10^{-20}$ & $1.1 \times 10^{-23}$ & $1.8 \times 10^{-20}$ \\
J1918$-$0642 & $2.57 \times 10^{-20}$ & $1.9\times 10^{-21}$ & $1.4 \times 10^{-22}$ & $3.7 \times 10^{-22}$ \\
\hline  
\end{tabular}
\tablefoot{Contributions due to accelerations in the Galactic potential, $\dot{P}_{\rm Gal}$, are calculated using equations in \cite{tsb+99}.}
\vspace*{1.5mm}
\end{table}