A horizontal, $25-$mm diameter cylinder is maintained at a uniform surface temperature of $35$C$.$ A fluid with a velocity of $0\mathrm{.}05$ m$/$s and temperature of $20$C is in cross flow over the cylinder. Determine whether heat transfer by free convection will be significant for $($i$)$ air, $($ii$)$ water, $($iii$)$ engine oil, and $($iv$)$ mercury.