\begin{table}%t2
\caption{\label{Table: columns}Total column densities, $N$, and abundances with respect to hydrogen,~$X$.}
%\centering
\par
\begin{tabular}{lcc} 
 \hline\hline \noalign{\smallskip}
Species $x$  	&$N$\tablefootmark{a} & $X$ = $N_x/N_{\rm H}$\\ 
  	& (\cmsq) & \\
\hline \noalign{\smallskip}
NH  &1.5 $\times$ 10$^{14}$	& 5.6 $\times$ 10$^{-9}$  \\  
NH$_2$	 	  	&8.0 $\times$ 10$^{13}$&3.0 $\times$ 10$^{-9}$  \\    
NH$_3$	 	 	  &	8.7 $\times$ 10$^{13}$ & 3.2 $\times$ 10$^{-9}$	\\   
NH$^+$  	  	& ${\la} 2 \times 10^{13}$  &  ${\la} 8 \times 10^{-10}$	  \\ 
\hline 
\end{tabular}
\tablefoot {\tablefoottext{a}{Calculated in  the velocity interval \mbox{11--54~\kms}.}}
\end{table}