PUBLICATION No. 102 of the Royal Meteorological Institute; of the Netherlands consists of a doctor's dissertation in Dutch by Miss Annie van Vleuten “On the Diurnal Variation of Terrestrial Magnetism” and two short papers in English from vol. xxvi. (1917). of the Proceedings of the Science Section of Kon. Ak. v. Wet. of Amsterdam. The dissertation which extends to 106 pages, contains numerous tables of diurnal variation data for the magnetic elements, and the corresponding Fourier coefficients for a number of stations, more especially for Pavlovsk, Sitka, Irkutsk, De Bilt, Cheltenham, U.S., Zi-ka-wei, Honolulu, Bombay, Buitenzorg, and Samoa, and for the group of years 1906–8. The Fourier coefficients, based on the data from these ten stations from the international quiet days, five a month, are used to furnish answers to the questions advanced in the two short papers in English: (1) Does the internal magnetic field to which the diurnal variation is partly ascribed depend on induced electric currents? (2) Do the forces causing the diurnal variation possess a potential? These are problems chiefly associated in England with the name of Prof. Schuster, to whose work there are many references, while abroad they have occupied, amongst others, Profs. Fritsche and Steinef. Schuster and Fritsche, using totally different observational data, separated the forces causing the diurnal variation into one set having a source external to the earth, and a second set having an. internal source. Schuster suggested that the second set arise from currents induced in the earth by the former set. Steiner, employing Fritsche's results, decided against Schuster's hypothesis. Miss van Vleuten's material,}s at once more homdgeneotis than Fritsche's, and more representative than Schuster's. She concludes that. while the terms of higher order accord pretty fairly on the whole with Schuster's hypothesis, this is not true of the principal terms of lower order. The natural inference is that the hypothesis is, at best, not a complete explanation of the phenomena. To the second question the answer obtained is that the forces causing the diurnal variation do not possess a potential; part, but only part, of the diurnal variation may be derived from a potential. Besides the main data mentioned above, data from a number of other stations are utilised, and there is, besides, a good deal of mathematical theory. While the publication makes most direct appeal to theorists, it contains much valuable information as to facts not otherwise readily accessible.