  We measured experimentally the static friction force $F_s$ on the surface of
a glass rod immersed in dry sand. We observed that $F_s$ is extremely sensitive
to the closeness of packing of grains. A linear increase of the grain-density
yields to an exponentially increasing friction force. We also report on a novel
periodicity of $F_s$ during gradual pulling out of the rod. Our observations
demonstrate the central role of grain bridges and arches in the macroscopic
properties of granular packings.
