XV. On the lunar-diurnal variation of the magnetic declination, with special regard to the moon's declination

Abstract

The hourly records of the magnetic declination, systematically kept at the Flagstaff Observatory at Melbourne, Victoria, during the period from the 1st of May 1858 to the 28th of February 1863, have been discussed with a view to determine the lunardiurnal variation to which this magnetic element is subjected. The results arrived at in the course of this discussion, eliciting, as I believe, facts hitherto unnoticed, induced me to think it important to have them brought before the Royal Society, with no other object in view but to direct the attention of scientific men to a subject of such vast import for the development of the science of terrestrial magnetism. The process adopted in reducing the observations, in order to eliminate the solardiurnal variation of the magnetic declination, is identical with the one generally adopted in such cases. The limit of disturbance was taken to be 3·61 minutes of arc, and all hourly directions which differed from their final normals by this value, or more, were consequently omitted from the record. This elimination of the larger disturbances having been effected, from every remaining reading (R) of the magnet’s direction the respective final normal (N) was subtracted, thus causing the residuals (R—N) to be devoid of the influence of the solar-diurnal variation. When the remainders are negative, i. e. when the normal exceeds the reading, the north end of the needle is to the west of its mean direction, and when positive the needle deviates with its north end towards the east of its normal mean. The magnetic declination being east at Melbourne, we perceive that the negative values denote a decrease, and the positive ones an increase, with respect to the normal value of this, magnetic element. The total number of obser­vations at command amounts to 38, 194, of which 4, 178 single readings were excluded from the discussion, on account of their being beyond the above-mentioned limit, and there remained only 34, 016 readings for the purpose of determining the lunar-diurnal variation. Of this number 15, 735 were observed in the months from April to September, and 18, 281 during the time from October to March.