\begin{table}%T1
   %%\centering
\par
      \caption{The gravitational force $F_{\rm g}$ and the
      Lorentz force $F_{\rm L}$ acting on a dust particle with a radius $a$
      have been compared under typical interplanetary conditions
      (at $1~ {\rm AU}$, solar wind with $v_{\rm sw} = 400 ~ {\rm km ~s^{-1}}$,
      magnetic field $B = 5 {nT}$, particles charged to $U = 10~ {\rm V}$;
      forces are given in Newton).}
         \label{tab2}
         \begin{tabular}{cccc}
              \hline\hline
            \noalign{\smallskip}
             $a$ (${\rm \mu m}$) & $F_{\rm g}$ (${\rm N}$) &
             $F_{\rm L}$ (${\rm N}$) &  ${F_{\rm L} \over F_{\rm g}}$ \\
\par
            \noalign{\smallskip}
            \hline
            \noalign{\smallskip}
            0.1 & $8.7 \times 10^{-20}$  & $1.6 \times 10^{-19}$ & $1.84$ \\
            1   & $8.7 \times 10^{-17}$  & $1.6 \times 10^{-18}$ & $1.84 \times 10^{-2}$ \\
            10  & $8.7 \times 10^{-14}$  & $1.6 \times 10^{-17}$ & $1.84 \times 10^{-4}$ \\
            100 & $8.7 \times 10^{-12}$  & $1.6 \times 10^{-16}$ & $1.84 \times 10^{-5}$ \\
            \noalign{\smallskip}
            \hline
       \end{tabular}
\end{table}