A sphere of density $\mathrm{d}$, specific heat capacity $\mathrm{c}$ and radius $\mathrm{r}$ is hung by a thermally insulating thread in an enclosure which is kept at a lower temperature than the sphere. The temperature of the sphere starts to drop at a rate which depends upon the temperature difference between the sphere and the enclosure. If the temperature difference is $\mathrm{\Delta T}$ and surrounding temperature is ${\mathrm{T}}_0$, then rate of fall in temperature will be [Given that $\mathrm{\Delta T}<<{\mathrm{T}}_0$]