$($a$)$ Estimate the permeability of a bed of sand, with uniform spherical particles, Dp $=1$ mm$,\backslash$epsi $=0\mathrm{.}3,$ according to Eq$.11\mathrm{.}21.$

$($b$)$How does this compare with the value for sand $($which is rarely spherical$)$ in Table $11\mathrm{.}1?$