  Aims : We present 24 synoptic maps of solar filaments, in which the average
unambiguous magnetic field vectors of 296 prominences were determined with
Pic-du-Midi observations between 1974 and 1982. This was the ascending phase of
cycle 21. Methods : The magnetic field was determined by interpreting the Hanle
effect, which is observed in the \ion{He}{i} D$_3$ line. Previous results for
the prominence field polarity and prominence chirality were applied to solve
the fundamental ambiguity. The measurements were averaged in each prominence
for accuracy reasons. Results : The result is twofold. First, alternating field
directions can be observed from one neutral line to the next. Second, a general
field alignment is found along a solar north-south field that is distorted by
the differential rotation effect. The numerical data for the prominences and
their magnetic field coordinates are provided as online material associated
with this paper.
