On the lunar‐diurnal variation of the magnetic declination at Zi‐ka‐wei

The September 1915 issue of the Journal, p. 139, contains an abstract of papers by Dr. S. Chapman on the lunar‐diurnal variation of the Earth's magnetism. The following notes, in which an analogous question is considered with less detail, may be of interest. In this discussion, we have limited ourselves to the consideration of the diurnal amplitude of the declination because this element is measured here every day photographically, and we are thus made independent of all computation. By diurnal maximum and minimum we understand that the elongations which take place at midday and toward sunrise, are meant. A larger value, which might occur at another hour, would be considered as belonging to another phenomenon.