II. On the 26-day period of the earth’s magnetic force

Abstract

The Astronomer Royal’s communication to the Royal Society on this subject ( suprà , p. 308) has drawn my attention to the investigation made by the Director of the Prague Observatory. Dr. Hornstein having remarked the uncertainty of the result for the time of the sun’s rotation as deduced from the movement of the spots in different zones on its surface, thought it would be desirable to consider other phenomena associated with the sun’s rotation; and the apparent connexion of the frequency of the solar spots with the amount of the magnetic oscillations induced him to seek for a period in the daily mean values of the magnetic elements. For this end he grouped the daily means of observations made at Prague and Vienna in 1870 in periods varying from 16 to 28 days; and subjecting the resulting means to calculation for the term a sin (θ + c ) in the usual formula of sines, he considered the most probable of the periods to be that for which a had the greatest value.