We present a method for measuring small deviations from axisymmetry of the potential of a filled gas disk. The method is based on a higher order harmonic expansion of the full velocity field of the disk. This expansion is made by first fitting a tilted-ring model to the velocity field of the gas disk and subsequently expanding the velocity field along each ring into its harmonic terms. We use epicycle theory to derive equations for the harmonic terms in a distorted potential. The phase of each component of the distortion can vary with radius. We show that if the potential has a distortion of harmonic number m, the velocity field as seen on the sky exhibits an m-1 and m+1 distortion. As is to be expected, the effects of a global elongation of the halo are similar to an m=2 spiral arm. The main difference is that the phase of the spiral arm can vary with radius. Our method allows a measurement of epsilon_pot sin(2 phi_2), where epsilon_pot is the elongation of the potential and phi_2 is one of the viewing angles. Using \hi data, one can probe the potential at radii beyond the stellar disk, into the regime where dark matter is thought to be the dominant dynamical component. The method is applied the spiral galaxies NGC 2403 and NGC 3198 and the harmonic terms are measured up to ninth order. We find epsilon_pot sin(2 phi_2) to be 0.064 +/- 0.003 for NGC 2403 and 0.019 +/- 0.003 for NGC 3198. More galaxies should be examined to separate viewing angle from elongation in a statistical way.

17 pages, 8 figures. To be published in Monthly Notices of the Royal Astronomical Society