$3).$ Air at $1$ atm abs pressure is enclosed between two horizontal plates and the bottom plate is hotter than the upper plate where L $=0\mathrm{.}6$ m and $=30$ mm$.$ The plates are $0\mathrm{.}4$ m wide. The plate temperatures are T$1=422$ K and T$2=394\mathrm{.}3$ K$.$ Calculate the heat$-$transfer rate across the air gap.