This paper deals with an extension of Mayaud's aa-index series (from 1868 to the present) for two solar cycles 1844--1868. The magnetic data sources are hourly declination readings at the Helsinki magnetic-meteorological observatory carried out in 1844--1880. The new data set provides a reliable and homogeneous series of declination for computations of equivalent aa-indices. For the time period 1868--1880 the linear correlation coefficient between the two indices was 0.96 on the monthly basis. Behavior of the geomagnetic activity during the course of the sunspot cycles 9--12 revealed similar periodicities and linear dependencies (Ohl's and Feynman's relationships) as later activity data analysis has shown. ¿ American Geophysical Union 1993