Microseisms of very great amplitude rise from the sea bottom, near the coast, and are always associated with rapid barographic perturbations, generally accompanied by cyclonic winds. The creation of the "fetch" is sufficient condition for the rising of microseisms; particularly persistent are those excited by coperiodal "significative waves". Only the microseisms excited by significative waves are propagated to very great distances. In the origin-zone of the microseismic perturbations, where the wavy systems attain their greatest development, rise microseisms of every periods, from the smallest ones to those coperiodal with the significative waves. A seismic station, near the origin-zone, for strong atmospheric perturbations, is then able to record simultaneously a whole scale of microseisms, which gives the seismogram a chaotic aspect. Only seismic stations, distant from origin-zone, record regular microseisms: those associated with significative sea waves. In other conditions, propagation distance of microseisms increases with increasing of coperiodal significative waves period. Microseisms are generally excited by waves which "feel the bottom": viz. are originated as interaction from water and bottom, on the rhythm of the primary frequency. The double frequency, which require a second order effect - in conformity with Longuet-Higgins theory -, can occur out to sea, in deep waters. Microseisms present no appreciable dispersion in their propagation, even at very great distances.