  We present numerical results for the magnetic susceptibility of a Heisenberg
antiferromagnetic spin ladder, as a function of temperature and the spin-spin
interaction strengths $J_\perp$ and $J_{||}$. These are contrasted with new
bulk limit results for the dimer chain. A fit to the experimental
susceptibility of the candidate spin-ladder compound vanadyl pyrophosphate,
(VO)$_2$P$_2$O$_7$, gives the parameters $J_\perp = 7.82$ meV and $J_{||} =
7.76$ meV. With these values we predict a singlet-triplet energy gap of
$E_{gap} = 3.9$ meV, and give a numerical estimate of the ladder triplet
dispersion relation $\omega(k)$. In contrast, a fit to the dimer chain model
leads to $J_1=11.11$ meV and $J_2=8.02$ meV, which predicts a gap of $E_{gap} =
4.9$ meV.
