champ moyen

OBS: (...) la moyenne des mesures observées à la même époque pendant 10 années.

OBS: (...) measurements of the intensity and direction of the field have been made over the earth's surface, and (...) on the basis of repeated measurements at certain stations, these have been reduced to a single instant of time (...) in such a way that variations with periods less than a few years are averaged out. These reduced measurements define the main field at the time chosen.