\begin{table}%T4
\caption{Integrated line ratios in the centre of M~51.}
\label{m51rat}
%\centerline
 {
\begin{tabular}{lcccc}
\hline\hline
\noalign{\smallskip}
Transitions & \multicolumn{2}{c}{Center} &  \multicolumn{2}{c}{NW Arm GMA-1} \\ 
            & Observed & Modelled$^{a}$   & Observed & Modelled$^{a}$ \\
\hline
$\co$ (1--0)/(2--1)$^{b}$  & $1.25\pm0.10$ & 1.28 & $1.40\pm0.20$ & 1.35 \\
$\co$ (3--2)/(2--1)$^{c}$  & $0.78\pm0.23$ & 0.75 & $1.02\pm0.16$ & 0.87 \\
$\co$ (4--3)/(2--1)$^{c}$  & $0.49\pm0.10$ & 0.41 & $0.44\pm0.08$ & 0.46 \\
\noalign{\smallskip}
$\co$/$\treizeco$ (1--0)$^{d}$ &    $7\pm1.5$  &  6.7 &    $8\pm1.5$  &  8.2 \\
$\co$/$\treizeco$ (2--1)$^{c,d}$ &  $6\pm1$    &  6.9 &  $8.7\pm1.2$  &  8.1 \\
$\co$/$\treizeco$ (3--2)$^{e}$ & $12.9\pm1.8$  & 12.2 & $12.5\pm1.7$  & 13.2 \\
\noalign{\smallskip}
CI/CO(2--1)$^{c}$   	   & $0.22\pm0.06$ &  -- & $0.24\pm0.08$ & --  \\
CII/CO(2--1)$^{f}$  	   & $0.46\pm0.09$ &  -- &      --      & --  \\
\hline
\end{tabular}}
\smallskip
Notes: $^a$ See Table~\ref{m51model}.  $^b$ Ratios estimated from data by
Nakai et~al. (\cite{Nakai94}), NRO at 16$''$ resolution; Garc\'ia-Burillo et~al. (\cite{Garcia93}b), IRAM at 22$''$.  $^c$ This Paper, JCMT at 21$''$ resolution.
$^d$ Ratios estimated from data presented by Rickard \& Blitz (\cite{Rickard85}),
NRAO 65$''$, Garc\'ia-Burillo et~al. (\cite{Garcia93}b), IRAM at 22$''$, Matsushita et~al.  (1997), NMA at 6$''$; and Paglione et~al. (\cite{Paglione01}), FRAO at 45$''$
resolution.  $^e$ This Paper; JCMT at 14$''$ resolution.  f. From
Crawford et~al. (\cite{Crawford85}) and Stacey et~al. (1991), KAO at 55$''$
resolution; consistent with ISO flux (Negishi et~al. \cite{Negishi01}); JCMT
convolved to 55$''$ resolution.
\end{table}