QUESTION:
In the photos of Pathfinder, the "wind sock" looks like a cone with its apex
pointing up - not at all like a traditional wind socks used at airfields.
How does it work?

ANSWER from David Mittman on December 16, 1997:
The wind sock is in fact a hollow open cone at the end of a short shaft. I
think most of the material is aluminum. It is supported by a universal joint
pivot connected near the center of the shaft, but slightly off the center of
gravity so that the cone hangs vertically when there is no wind.  When the
wind blows, the cone rises. The outside of the cone is brushed aluminum, and
the inside is painted black, so that the camera can tell which way the cone
is tilting.

The atmosphere of Mars is too thin to inflate a conventional wind sock, but
the balanced sock described above does the job (just). Unfortunately the
threshold of this device is about 6-7 m/sec. During the Pathfinder mission
winds only occasionally exceeded this threshold.