  We report experimental results for convection near onset in a thin layer of a
homeotropically aligned nematic liquid crystal heated from below as a function
of the temperature difference $\Delta T$ and the applied vertical magnetic
field $H$ and compare them with theoretical calculations. The experiments cover
the field range $8 \alt h \equiv H/ H_{F} \alt 80$ ($H_F =$ is the
Fr\'eedericksz field). For $h$ less than a codimension-two field $h_{ct} \simeq
46$ the bifurcation is subcritical and oscillatory, with travelling- and
standing-wave transients. Beyond $h_{ct}$ the bifurcation is stationary and
subcritical until a tricritical field $h_t= 57.2$ is reached, beyond which it
is supercritical. The bifurcation sequence as a function of $h$ found in the
experiment confirms the qualitative aspects of the theoretical predictions.
However, the value of $h_{ct}$ is about 10% higher than the predicted value and
the results for $k_c$ are systematically below the theory by about 2% at small
$h$ and by as much as 7% near $h_{ct}$. At $h_{ct}$, $k_c$ is continuous within
the experimental resolution whereas the theory indicates a 7% discontinuity.
The theoretical tricritical field $h_t^{th} = 51$ is somewhat below the
experimental one. The fully developed flow above $R_c$ for $h < h_{ct}$ is
chaotic. For $h_{ct} < h < h_t$ the subcritical stationary bifurcation also
leads to a chaotic state. The chaotic states persist upon reducing the Rayleigh
number below $R_c$, i.e. the bifurcation is hysteretic. Above the tricritical
field $h_t$, we find a bifurcation to a time independent pattern which within
our resolution is non-hysteretic.
