\begin{table}%t4
%%\centering
\par
\caption{\label{comp} Comparison of model predictions for diffuse and dense clouds with the observed abundances (compared to $2N(\rm{H_{2}})+N(\rm{H})$) of CH and CH$_{2}$ ($X$(CH) and $X$(CH$_{2}$)). The diffuse cloud model is for the line of sight to $\zeta$~Per \citep{vandishoeck86}: vDB86. The quoted density and temperature are for the centre of the cloud in the model. The dense cloud models are steady state values corresponding to the gas phase "new standard model'' of \citet{lee}: L96 and the gas-grain model of \citet{ruffle}: RH01 including photochemistry (P1/A at 10$^{8}$~yr). The PDR model is for a density of 10$^{5}$~cm$^{-3}$ and radiation field $\sim$650--900 times the average interstellar value \citep{jansen95}: J95. The observational results derived from our ISO measurements are shown as well as previous results from \citet{lyu} and \citet{hollis}.}
%\leavevmode \footnotesize
%\centerline
 {\begin{tabular}[!t]{lcccccc}
\hline
\hline
                 &                     & Density     & Temperature &  $X$(CH)    & $X$(CH$_{2}$) & $[{\rm CH}/{\rm CH}_2]$ \\
                 &                     &(cm$^{-3}$)  & (K)         & ($10^{-8}$) & ($10^{-8}$)   &  \\
 \hline
Model        & Diffuse (vDB86)         & 500             & 25 & 1.6       & 2.4        & 0.67 \\ 
predictions: & Dense    (L96)          & $10^3$          & 10 & 0.25      & 1.6        & 0.018  \\ 
             & Dense      (L96)        & $10^5$          & 10 & 0.0007    & 0.039      & 0.16  \\ 
             & Dense   (RH01)          & $2\times10^{4}$ & 10 & 0.1       & 1.2        & 0.08 \\ 
             & PDR    (J95)            & 10$^5$          &    & 1.4       & 0.7        & 2.0  \\ 
 \hline
Results from & Sgr B2 Line of Sight           &            &         & 0.6--3    & 0.2--1.1   & $2.7\pm0.5$   \\
this work:   & \object{Sgr~B2}                &                     &         & 0.05--0.2 & 0.02--0.07 & $2.7\pm0.5$   \\ 
             & \object{W49~N} 37~km~s$^{-1}$           &            &         &           &            & $>$1.4 \\  
             & \object{W49~N} 61~km~s$^{-1}$           &            &         &           &            & $>$0.8\\
             & \object{Sgr A$^{\ast}$}                 &            &         &           &            & $>$2.6\\  
 \hline
Previous     & $\zeta$~Oph                    &            &          & 1.8$^{b}$   & 4.3$^{a}$  & 0.4$^{b}$ \\ 
observational& HD154368                       &            &          & 1.9$^{d}$   & 0.79$^{c}$ & 2.4$^{d}$ \\ 
results:     & Orion-KL                       & $\sim$$10^5$ & 100--200 & 0.06$^{e}$  & 0.4$^{e}$  & 0.1$^{e}$ \\ 
             & W51~M                          & $\sim$$10^4$ & 100--200 & 0.007$^{e}$ & 0.07$^{e}$ & 0.1$^{e}$ \\ 
\hline
\end{tabular}}
\par
$^{a}$ Combining $N({\rm CH}_2)$ from \citet{lyu} with a hydrogen column density as quoted in \citet{vandishoeck86}.\\
$^{b}$ Based on $N({\rm CH})=2.5 \times10^{13}$~cm$^{-2}$ from \citet{vandishoeck89}.\\
$^{c}$ Combining $N({\rm CH}_2)$ from \citet{lyu} with a hydrogen column density from \citet{1996ApJ...465..245S}.\\
$^{d}$ Based on $N({\rm CH})=8 \times10^{13}$~cm$^{-2}$ from \citet{vandishoeck89}.\\
$^{e}$ Taking values as quoted by \citet{hollis} -- but note that $X$(CH) could be significantly higher -- see text.\\
\end{table}