A brief introductory discussion is given of the importance of internal waves for theoretical and applied oceanography. The temperature series obtained at four oceanographic stations in the North Atlantic are subjected to harmonic analysis and to the expectancy test in order to determine the possible presence of periodic variations coinciding with the length of the principal lunar semidiurnal tide. Only in one case is it possible to regard the computed harmonic coefficients as representative of tidal effects, with a reasonable degree of assurance while in the other three cases any possible tidal effects are either more or less strongly obscured by random variations or absent. Because of the shortness of the existing oceanographic series a completely statisfactory decision concerning the presence or absence of significant variations of tidal character in the observations is impossible. It is pointed out that oceanographic serial observations of longer duration made simultaneously at different stations would lead to substantial progress in the study of internal waves.