\begin{table}%T3
%\centering
\par
\caption{Comparison of rms values of three instruments.}
\label{tab3}
\begin{tabular}{lcrcc}
\hline\hline
Observatories&Years&$m$&$\Delta '$&$\Delta ''$\\
&&&(G)&(G)\\
\hline
CrAO minus WSO&1975--2001&712&0.47&0.50\\
MWO minus WSO&1975--1982&1442&0.60&0.64\\
SSO ~~minus WSO&1982--1993&256&0.51&0.62\\
\hline
\end{tabular}
\end{table}