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It is known to those who have devoted attention to thejiip of the magnetic needle, in surveying or otherwise, that there is a diurnal variation or disturbance of its action, as was clearly set forth in the letter which appeared on this subject in our columns last week. We find some very interesting information on this point in a letter recently read before the Royal Belgian Academy of Science, by the secretary, M. Quetelet, and received from him by the Swedish philosopher, M. Hansteen. The latter states in his letter that, from observations made in four summer months with a dipping needle and unifilar and bifilar horizontal needles, he has come to the conclusion that the diurnal variation, observable in magnetic phenomena, is produced by a feeble perturba-tive force which turns around the horizon from east to west in twenty-four hours. " When this force proceeds to the south, the horizontal intensity diminishes, the inclination .augments, and the declination has its mean value (about ten hours before mid-day); when it proceeds to the north, the horizontal intensity increases, the inclination diminishes, and the declination assumes its mean value, which takes place about an hour before sunset ; when it proceeds towards the west or the east, the respective declination augments or diminishes (one hour after mid-day, eight hours before mid-day or mid-night.)" The regular inclination or dip of the needle, which is now decreasing, will reach its minimum, Hansteen thinks, in Western Europe in 1878, and it has already reached it in Siberia. It was at its maximum in Europe in 1678, thus indicating a period of two hundred years between the extremes in the dip of the needle. The decrease has proceeded at the rate of 2-316, or about two minutes and a third per annum.
