  We consider the 3D equation $u_{yy} = u_{tx} + u_yu_{xx} - u_xu_{xy}$ and its
2D reductions: (1) $u_{yy} = (u_y+y)u_{xx}-u_xu_{xy}-2$ (which is equivalent to
the Gibbons-Tsarev equation) and (2) $u_{yy} = (u_y+2x)u_{xx} + (y-u_x)u_{xy}
-u_x$. Using reduction of the known Lax pair for the 3D equation, we describe
nonlocal symmetries of~(1) and~(2) and show that the Lie algebras of these
symmetries are isomorphic to the Witt algebra.
