$4\mathrm{.}2\mathrm{.}18.$ A tube subjected to torsion is of rectangular form, the outside dimensions being $50$ mm x $25$ mm and the thickness $1\mathrm{.}6$ mm$.$ Show that for calculating both shear stress and angle of twist such a section may be treated as practically equivalent to a thin circular tube in which the mean radius is $2$A$/$S and with a thickness S$^2$t$/4\backslash$pi $\backslash$Alpha $,$ where A is the cross$-$sectional area, S the perimeter and t the thickness of the non$-$circular tube. If the material is brass $($G$34,500$ N$/$mm$^2)$ calculate the safe torque and maximum angle of twist per meter if the shear stress is not to exceed $27\mathrm{.}5$ N$/$mm$^2.$