\begin{table}%t5
\caption{The compared clocks, the value of $\frac{\dot \alpha}{\alpha_0}$, its corresponding error in units of $10^{-15}~{\rm yr}^{-1}$, the time interval for which the variation was measured, and the reference. }\label{clocks}
\small%\centerline
 {
\begin{tabular}{cccc}
\hline \hline
%&&& \\[-8pt]
Frequencies&$\frac{\dot \alpha}{\alpha_0} \pm \sigma
\left[ 10^{-15}~\rm{yr}^{-1}\right]$& $\Delta t [\rm{yr}]$&Reference \\ 
%&&& \\[-8pt]
\hline
Hg+ and H maser& $0.0 \pm 37.0$ & 0.38 & (1) \\ 
Cs and Rb & $4.2 \pm 6.9 $& 2 & (2)\\ 
Cs and Rb & $-0.04 \pm 1.60 $ & 5 & (3) \\ 
Hg and Cs & $ 0.0 \pm 1.2 $ & 2 & (4) \\ 
H and Cs & $1.14 \pm 2.25 $ & 5 & (5) \\ 
Yb and Cs & $-0.58  \pm 2.1 $ & 2.8 & (6) \\
\hline
\end{tabular}}
\medskip
References: (1) \citet{PTM95}; (2) \citet{Sortais00}; (3)~\citet{Marion03}; (4) \citet{Bize03}; (5) \citet{Fischer04}; (6) \citet{Peik04}.
\end{table}