Many important kinds of curves (like equipotentials) in plane problems of physics and engineering are determined by an equation of the form u(x,y) = C, where u(x,y) is a harmonic function and C a constant. Here a solution of that equation is suggested (on the basis of previous analogous results for analytic functions). This solution contains a parameter varying along the curve under consideration and requires the use of convergent quadrature rules for the numerical evaluation of the integral appearing in it. An application to a simple problem of potential theory (e.g., heat transfer) is also made, and points of lines of heat flow are determined by the present method. Finally, a generalization of the present results to the complicated equation of the theory of caustics in dynamic plane fracture mechanics [where u(x,y) is no longer a harmonic function] is made.