Translations:Ray equation for velocity linear with depth/7/es

${\begin{aligned}\mathbf {u=} {\frac {d\mathbf {r} }{ds}}&{\rm {=}}\left({\frac {dx}{ds}},{\frac {dy}{ds}}\right){\rm {=}}\left({\rm {\ sin\ }}\theta {\rm {,\ cos\ }}\theta \right).\end{aligned}}$

(28)