The paper gives an account of the various sources of error which have been encountered during the development of the Bellini-Tosi system of wireless direction-finding as an aid to navigation in the British mercantile marine. These errors can be isolated and their causes removed one by one.The application of the instrument to the purposes of navigation is also dealt with, and the possible errors, external to the ship, which may be encountered- are considered.A table is attached showing the high degree of accuracy which has now been attained.