II. On the lunar diurnal variation of magnetic declination at the magnetic equator
This variation, first obtained by M. Kreil, next by myself, and afterwards by General Sabine, presents several anomalies which require careful consideration, and especially a careful examination of the methods employed to obtain the results. The law obtained seems to vary from place to place even in the same hemisphere and in the same latitude, and this to such an extent, that, for example, when the moon is on the inferior meridian at Toronto it produces a minimum of westerly declination; while for the moon on the inferior meridian of Prague and Makerstoun in Scotland it produces a maximum of westerly declination. No two places have as yet given exactly the same result; though the result for each place has been confirmed by the discussion of different periods. In order to obtain the lunar diurnal action, it has been usual to consider the magnetic declination at any time as depending on the sun’s and moon’s hour-angles and on irregular causes. Thus, if at conjunction, H 0 be the variation due to the sun on the meridian, and h 0 be that due to the moon on the meridian, H, the variation for the sun at 1 h , h 1 for the moon on the meridian of 1 h , and so on; it is supposed that we may represent the variations for a series of days by the following expressions, where the nearest values of h to the whole hour-angles are given:-