18.4 Cast-iron cannonballs used in the War of 1812 were occasionally heated for some extended time so that, when fired at houses or ships, they would set them afire. If one of these, the so-called "hot shot," were at a uniform temperature of 2000 F, how long after being exposed to air at 0 F with an outside convective heat-transfer coefficient of 16 Btu/h ft F, would be required for the surface temperature to drop to 600 F? What would be the center temperature at this time? The ball diameter is 6 in. The following properties of cast iron may be used: k= 23 Btu/h ft F Cr=0.10 Btu/lb F P = 4601b/