IV. Magnetic observations made at Stonyhurst College Observatory, from April 1863 to March 1870

Abstract

Results of Seven Years' Observations of the Pip and Horizontal Force . In a paper published in the Philosophical Transactions for 1863, the President of the Royal Society enters into a discussion of a six years’ series of the Kew Magnetic Observations, with the view of ascertaining whether the sun’s position with reference to the earth really produces a sensible semiannual inequality in the terrestrial magnetic elements. The probability of the existence of such a disturbing cause, founded on a comparison of the monthly determinations of the Dip and Horizontal Force taken at Hobarton and at Toronto, was made much more evident by Sir Edward Sabine’s discussion of the Kew observations; and it was from a desire of bringing more data to bear upon this important question that the reduction of the seven years’ observations, just completed at this observatory, has been undertaken. With this object in view, it was considered of great importance to adhere closely to the plan laid down by Sir Edward Sabine, and to present the results, for the sake of comparison, in three Tables. The first contains the mean monthly determinations of the elements, with their deduced mean values and secular variation, the second presents a view of the semiannual inequality, and the third gives the residual errors and the consequent most probable errors of any single observation and of the deduced mean values of the elements. Magnetic observations were first taken at Stonyhurst in 1858, but the continuous, series of monthly determinations of the Dip, Declination, and Intensity were only commencedin March 1863. The same instruments, i. e . a dip-circle by Barrow and a Jones unifilar, have been used throughout the whole seven years. These instruments were both tested at Kew before being sent to this observatory, and the dip-circle was again examined there last January with most satisfactory results. The constants of the vibration-magnet were determined by Mr. W else, and are given in the Proceedings of the Royal Society for February 1865. In calculating the value of the Horizontal Force, it has never been found necessary to apply any correction for the arc of vibration, which has always been small, nor for the Frodsham chronometer, whose rate has never exceeded 2 s per day.